in usual using, they are not so quite differnt. but, grammatical differnt have. では-conjunction は-particle(almost same as 'be' verb) these differnces in my feeling as native Japanese, “では” makes differnt from others more clear than “は” 「この学校【では】…」 （if other schools have no test　※【, but） this】 school has exams before the summer vacation begins. 「この学校【は】…」（just explanation ; ）this school has exams before the summer vacation begins. its up to just my feeling. so may be its not correct.