(Hopefully, you are using a Windows OS);
Depends how smart he is and how you "deleted" your history.
a.) Who's network was it on? If it was the school's, he could probably check sites you've visited.
In that case, you're screwed.
*IF you were doing it on a different network, you still might have a slight chance of getting away with web surfing.
b.) If you hit Ctrl+H and then deleted your history, there are still tracks left behind.
You have to find a way to deleted your cookies via a program (such as "CCleaner") or via your default browser.
-You need to know exactly where all of your extra temporary internet files are stored on the hard drive.
TIP: Navigate to the %appdata% folder, it will show you most of the sites you've loaded. This contains data such as saved progress in games; if you've ever played them on that computer. If so, just delete the folder that contains the "unallowed" content, then clear your recycling bin.