I don't think many players actually have actually graduated from college. Lisa Raymond and Laura Granville did a couple of years at Florida and Stanford respectively before turning pro but they didn't graduate. The only one I can think of who finished college is Debbie Graham, but there are a few others. A lot of the Americans finished high school, e.g. Lindsay Davenport, Venus and Serena, Corina Morariu, Alex Stevenson. Some of the Russians, e.g. Elena Dementieva, continue their schooling while they are on tour, mainly doing things like languages by correspondence course. There are also some countries that tie funding to education, like Australia as someone said, and I think Britain does as well.
I suppose it's quite difficult to go to school and play tennis at a high level because tennis demands a lot of time. It would always be at the back of your mind that you could do all the education thing when you had finished playing. Also, there just isn't enough competition. Even at college you're not playing people of the same kind of level that you would be doing if you were on tour - when Lisa Raymond played NCAA she lost something like 1 match in the whole 2 years she played - so you aren't getting the same kind of experience.
On the other hand, imo, players should finish high school because it would be difficult for someone to go back and finish that after their playing career ended. We may see more people finishing that level anyway, even if it's by correspondence, because of the age restrictions. You also have to remember that not all these kids with talent who sacrifice school for a tennis career will ever have a tennis career. It's a big gamble to throw away your education, which can affect your whole life, for something that might never happen. What happens to them if they get injured, don't enjoy it, aren't as good as everyone thought? If they at least have a high school diploma then they have a chance to do something else. Even if they do make it, what are they going to do after they retire? 50 years is a long time to just sit around, counting your money and watching video loops of your past triumphs