I think you should follow Dog's advice and not say anything further to or about this man to anyone, and you should have absolutely no contact of any kind with him. If served with a complaint, hire an attorney.
There is no way to tell if you would lose a defamation action if this guy decides to file suit against you. He will probably only file suit if he finds your comments have adversely affected his relationships with the teachers or staff or administrators or students on campus to the extent that he cannot function in his job effectively, or he loses his job.
All this man needs to do to win a defamation action against you is to prove the statements you emailed to various and assorted people on campus were defamatory, that you were at least negligent with respect to the making of these statements, and his reputation has suffered as a result.
Your defenses to a defamation suit would be limited to truth, substantial truth, opinion or privilege. The statements you made are not covered by any privilege, and opinion that implies defamatory fact is not protected opinion (although the wording of the emails should be reviewed carefully, to see if your statements can pass as opinion). Using truth to support your claim of sexual harassment can be defeated with the school's finding of no guilt.
Therefore, you are probably left with just a defense of substantial truth. Substantial truth as a defense can fly when there are minor factual inaccuracies, but the gist or the sting of the statement remains true despite these inaccuracies.
In the cases where this defense has been used successfully, the minor inaccuracies are minor - saying someone tested positive for cocaine when they really tested positive for heroin; saying someone was beaten 10 times when they were really beaten 8. The gist of the statements are true, and the effect of the statements are not changed substantially by the inaccuracy.
In your case, you are claiming sexual harassment based on evidence that the school has already discounted, however. You claim sexual harassment when, essentially, there was no finding of sexual harassment. Although it is possible that a good attorney could help you use this defense effectively with the documentation you have (and I agree that, based on what you report here, that the man's actions are a bit questionable), it is not a sure thing by any means.
To combat a defense of substantial truth, the man would need to have convincing evidence of the falsity of your statements. Although the school's finding of no guilt may be convincing enough for the court, balanced against your documentation it could be a toss up. In the event of a "toss up," and to your favor, courts give the benefit of any doubt to defendants.
I think you may benefit from having an attorney review your emailed statements now. Go over with the attorney all that led up to your sending of the emails. This can give you a clearer picture of what you may face and how to address any suit that may arise.
Again, the man may decide to drop the matter, especially if he is finding support from his employer/the school. He could chalk it all up to an irrational student over-reacting to innocent actions. He may never seek legal counsel.
If he does, however, and he sues, you must deal with it. For dealing with it, you should have an attorney.