This topic has been discussed on many threads. Here is the official Tennis Recruiting position on the topic...
(1) We want all kids to have correct graduation years, but there is no good source for that information. We make educated guesses on kids - which is what leads to the clearly-marked "provisional" graduation years on profiles. When players/parents link to their profile, they can confirm/update their graduation years.
(2) We have policies in place that make it difficult for people to modify a confirmed graduation year. Modification involves contact from Tennis Recruiting personnel - and in many cases we require documentation if they are switching to a younger class year.
(3) We invite our users to contact us about kids suspected of having an improper graduation year. We follow those up. Send any comments to firstname.lastname@example.org
(4) If a player is "old" for their grade, we may request documentation from a school, but we allow it. There are many situations that lead to 19-year-old seniors - we are not in the business of tracking these down to determine the reasons behind the classification. If a player can provide documentation for a graduation year, we will rank him with that graduation year.
I hope this helps.