However, the under-21ers that do drink, have to do so in manners that are more risky.  They have to drink either alone or in small groups, usually behind closed doors where it is hard or impossible for sober/older/mature/aware people to monitor and intervene if it becomes necessary.

At my wife's college, anyone under the age of 21 caught drinking can get in huge trouble, ranging from several-hundred dollar fines, judicial board,  etc...  Does this stop them from drinking?  No, they just do it in their dorm room with a locked door, often alone (like getting 'warmed up' before a party) or with one or two friends who are also drinking (and usually also underage).  This has led to a lot of problems, including a girl who downed some pills and then spent the rest of the night drinking alone in her room the week before finals, and ending up dead as a result.

It's also more common for students to leave campus on weekends to go party at neighboring colleges, which can be more dangerous for a number of reasons.  The college can say "hey, look how many people we've caught drinking, we're doing a good job" but in reality they're making things more dangerous.

Under-21ers also have to find more subversive ways to acquire alcohol, or drink with people they don't necessarily know well or trust.  I know of a young woman I went to high school with who went out to drink with some older guys, ended up getting passed-out drunk, and was raped by several of them.  Would she have been as inclined to go out drinking with a bunch of people she hardly knew if she could have just gone to a bar or something with her friends?  Probably not.