I agree with simply esl. split them up into more age appropriate groups maybe even at the cost of mixing the levels. mixed levels will be much easier to deal with than the huge difference in ages. if it was just a few years it wouldn't be a problem but considering your talking about a student who just got out of kindergarten versus one that is about to go to university, the gap is too big.
btw, where are you in missouri? i grew up in st. louis and went to uni in springfield...
eric