My site is a forum for my university.So most of the people who visit my site is the students in my university.But there is a problem.Many students surf the internet from the Local area network which opend by my college.So their IP are the same.When they click the ads,whether google considered it's cheats?

it is possible that google will consider it's cheats ,however you can email them for more imformation or explain somthing

Try to avoid logging in from IPs other than your own at home.