Our school is bigger than tjeirs,isn't it?
你问的应该是反义疑问句吧.
反义疑问句遵循的原则是：前面否定（包括seldom、few、little、never等词；但unhappy等含有前后缀的词不算否定哦）,后面就肯定 .前面肯定,后面就否定.最后一个单词数错了Our school is bigger than theirs就是反义句