1、他是胖子He is fat／他不是胖子He is not fat isn’t fat＝不胖 2、correct 谁上小学or谁读小学

Isn't fat = 不是胖/不胖 In this case, 不胖is more suitable. you can use 不是胖 but it doesn't seem like complete sentence. You need longer expression. e.g. He isn't fat but he is short. 他不是胖，而是矮小。 谁去上小学 is correct. Who is going to study in primary school. 谁去小学 is a bit incorrect but it still can be meant by: Who is going to primary school.