Drummers are known for drooling over all the place. So this is just a play on the word "get" - "get on his IQ test" here could mean to "obtain a score on the test," or more literally, "to cause something to be put/placed on something." (think "get on the bus"). So while drumming, the drummer's saliva flew everywhere, which it got onto his IQ test sheet... Not a terribly funny joke, if you ask me.