1.This is our classroom,it is on the second floor.
改为 This is our classroom which is on the second floor.
或者 This is our classroom that is on the second floor.
2.Today is may.24th,he was born on may .24th.1999/it was born on may .24th.1999
改为 Today is May.24th on which he was born in 1999.
3.My father has a bus,we go to school by it.
改为 My father has a bus by which we go to school.
多谢楼上提醒!