summer school

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An academic session held during the summer, chiefly for supplementary and remedial study.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of various academic sessions held in the summer at schools or universities when the usual students are away; either for supplementary or remedial study or as part of distance learning.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an academic session during the summer; usually for remedial or supplementary study