"If overseas students studied in our school, the would definitely be paid a lot of attention to." Just a few small changes to make it correct. "If overseas students studied in our school, they would definitely be paid a lot of attention." As a native speaker, I'd word it this way myself: "If overseas students studied at our school, they would definitely be given a lot of attention."

It's always a good idea to avoid the passive, if possible, so I'd say: "If foreign students attended our school, they would definitely receive a lot of attention."