I didn't want to report it because I may not be correct. I translated this as "A school project" as in a project related to or that is a part of schoolwork. The translation of "School's project" implies a project undertaken by the school itself. I hope someone else can clarify.
Without context it could be either meaning but the most common use would be "(a) school project" with "a" being optional. "A school project" should be marked correct . So report it if it wasn't accepted
Is this a project a school undertakes; a project to build a school; and/or a project that a student does for school?
Without clues from the context just translate the words as best you can. Sometimes the editors/compilers want articles with nouns but not always.
"a school's project" has been marked wrong! However the phrase stands for "project of school" which is same as "school's project"!