If you know of other schools that are not listed here, please let me know. Like in previous posts (Summer 2016, Winter 2016/ 2017, Summer 2017, and Winter 2017 / 2018), I may add schools with past deadlines as reference for readers looking for schools in the next years.
PS: For schools after September, check the Winter 2018 / 2019 post
NATCOR Heuristics & Approximation Algorithms
April 9-13 (deadline not posted)
Nottingham, England
International Spring School on High Performance Computing
April 23-27 (deadline: February 12)
San Sebastián, Spain
* Included on January 29
Optimal Transport: Numerical Methods and Applications
May 7-11 (deadline: March 10)
Como, Italy
International Spring School on Integrated Operational Problems
May 14-16 (deadline: March 31)
Troyes, France
Complex Networks: Theory, Methods, and Applications
May 14-18 (deadline: February 18)
Como, Italy
* Included on February 19
Summer School on Mathematics in Imaging Science
May 28 – June 1 (deadline: February 14)
Bologna, Italy
1st International Summer School on Artificial Intelligence and Games
May 28 – June 1 (deadline: January 31 for early registration)
Chania, Greece
* Included on January 29
VeRoLog PhD School on Vehicle Routing Problems
June 1-2 (deadline: January 31)
Cagliari, Italy
NATCOR Convex Optimization
June 4-8 (deadline not posted)
Edinburgh, Scotland
Association for Constraint Programming (ACP) Summer School 2018
June 4-8 (deadline not posted)
Jackson, WY, USA
* Included on February 19 by suggestion of Serdar Kadioglu
School on Graph Theory
June 11-15 (deadline: May 4 for early registration)
Sète, France
* Included on March 5
8th Lisbon Machine Learning School
June 14-21 (deadline: March 16)
Lisbon, Portugal
Summer Institute in Computational Social Science
June 17-30 (deadline: February 19)
Durham, North Carolina, USA
2018 Gene Golub SIAM Summer School on Inverse Problems: Systematic Integration of Data with Models under Uncertainty
June 17-30 (deadline: February 1)
Breckenridge, Colorado, USA
2nd International Workshop on Bilevel Programming (including mini-courses)
June 18-22 (deadline: April 15 for early registration)
Lille, France
Machine Learning Summer School
June 18-30 (deadline: February 20)
Buenos Aires, Argentina
The Data Incubator Data Science Fellowship
June 18 – August 10 (deadline not posted)
New York / San Francisco Bay Area / Seattle / Boston / Washington DC, USA, or online
* Included on January 29 by suggestion of Heitor H. Arakawa
International Conference on Automated Planning and Scheduling (ICAPS) Summer School
June 24-29 (deadline: March 23)
Delft, The Netherlands
DTU CEE Summer School 2018: Modern Optimization in Energy
June 24-29 (deadline: March 18)
Copenhagen, Denmark
* Included on February 2 by suggestion of Nicola Secomandi
Eötvös Loránd University Summer School in Mathematics: Introduction to Graph Limits
June 25-29 (deadline: April 30 for early registration)
Budapest, Hungary
2018 Summer School on “Operations Research and Machine Learning”
June 25-29 (registrations closed)
Fréjus, France
Summer School on Hyperbolic Polynomials, Sums of Squares and Optimization
June 25-29 (deadline: April 1)
Atlanta, GA, USA
* Included on February 19
15th International Conference on the Integration of Constraint Programming, Artificial Intelligence, and Operations Research (CPAIOR) Master Class
June 26 (deadline not posted)
Delft, The Netherlands
* Included on January 29 by suggestion of Willem-Jan van Hoeve
The Second Annual JuMP-dev Workshop
June 27-29 (deadline not posted)
Bordeaux, France
* Included on February 5
2nd School on Foundations of Programming and Software Systems: Logic and Learning
July 1-6 (deadline: April 15 for early registration)
Oxford, UK
* Included on March 22
EURO PhD School on Sustainable Supply Chains 2018 (preannouncement link)
July 1-7 (deadline not posted)
Wageningen & Amsterdam, The Netherlands
8th PhD School in Discrete Mathematics
July 1-7 (deadline: June 10)
Rogla, Slovenia
* Included on February 28
NATCOR System Dynamics
July 2-4 (deadline not posted)
Coventry, England
Summer School on Algorithms and Lower Bounds 2018
July 6-9 (deadline: April 15)
Prague, Czech Republic
* Included on February 2
ACM Special Interest Group on Genetic and Evolutionary Computation (SIGEVO) Summer School (website under construction)
July 13-19 (deadline not posted)
Kyoto, Japan
Prague School on Discrete Mathematics 2018
July 16-20 (deadline: March 30)
Prague, Czech Republic
Metaheuristics Summer School
July 21-25 (deadline: April 15)
Taormina, Italy
2nd International Summer School on Deep Learning 2018
July 23-27 (first deadline: February 14)
Genova, Italy
* Included on January 29
EURO PhD Summer School on MCDA/M (Multiple Criteria Decision Aiding / Making)
July 23 – August 3 (deadline: January 31)
Chania, Greece
8th Summer School on Imprecise Probabilities: Theory and Applications
July 24-28 (deadline: March 31)
Oviedo, Spain
Fundamentals of Data Analysis TRIPODS Madison Summer School 2018
July 24-28 (deadline not posted)
Madison, WI, USA
* Included on March 10
Deep Learning and Reinforcement Learning Summer School 2018
July 25 – August 3 (deadline not posted)
Toronto, ON, Canada
* Included on February 19
Argonne Training Program on Extreme-Scale Computing
July 29 – August 10 (deadline: February 28)
St. Charles, IL, United States
4th Algorithmic and Enumerative Combinatorics Summer School 2018
July 30 – August 3 (deadline: June 15)
Hagenberg, Austria
The Cornell, Maryland, Max Planck Pre-doctoral Research School 2018: “Emerging Trends in Computer Science”
August 7-12 (deadline: February 7)
Saarbrücken, Germany
Uncertainty Quantification Summer School
August 8-10 (deadline not posted)
Los Angeles, CA, USA
* Included on May 15
DIMACS/TRIPODS/MOPTA Summer School
August 10-12 (registration closed)
Bethlehem, PA, USA
* Included on May 15
Hausdorff School on Combinatorial Optimization
August 20-24 (deadline: April 30)
Bonn, Germany
Network Modeling for Epidemics
August 20-24 (deadline: May 1)
Seattle, WA, USA
* Included on March 12 by suggestion of Emily Tucker
SYNERGY Summer School on Efficient Multi-Objective Optimisation
August 27-31 (deadline: May 31 for early registration)
Ljubljana, Slovenia
Summer School on Statistical Relational Artificial Intelligence
August 27-31 (deadline not posted)
Ferrara, Italy
* Included on April 13
Data Science Summer School
August 28 – September 1 (deadline: April 20)
Paris, France
CPSE Summer School 2018: Optimisation Under Uncertainty
September 3-7 (deadline: July 31 for early registration)
London, England
* Included on May 18
ALOP Summer School on Mixed-Integer Nonlinear Programming
August 13-16 September 10-12 (deadline: May 30 June 15 for travel support application)
Trier, Germany
* Included on May 15, updated on June 12
FoMICS-DADSi Summer School on Data Assimilation
September 11-15 (deadline: June 30 for abstract submission)
Lugano, Switzerland
* Included on June 12
Fall School: Order and Geometry
September 14-17 (deadline: July 31)
Sauen, Germany
* Included on June 12
Graph Drawing 2018 PhD School: Recent trends in Graph Drawing and Network Visualization
September 24-25 (deadline: August 28 for early registration)
Barcelona, Spain
* Included on June 12
NATCOR Forecasting & Predictive Analytics
September 24-26 (deadline not posted)
Lancaster, England
Summer School in Algebraic Statistics
September 24-28 (deadline: June 30 for room application and/or travel support)
Tromsø, Norway
* Included on June 12