Why would have to lie? All you have to say is "My son/daughter will not be attending school today". If they ask why - tell them the truth. Mental Health. One day is not going to make or break a person. If any of you saying that it is a horrible thing to do, can honestly say that you have never called in sick to work when you weren't, or played hooky as a child, then you are a better person than I. But, give the kid some slack. HE IS A SENIOR!!!! He has earned this right. It is better than him having dropped out in freshman year, after skipping the first 5 months, and doing nothing for the rest of his teenage years but go back and forth to jail. This would be my brother. Look, at least he told you he is not going, rather than just skipping...hiding out somewhere until you left for work. He probably would have faked sick Monday morning anyway, and you would have ended up calling in, regardless. Let him do it.
And, another can of worms - a co-ed party/sleepover is not a big deal. It does not automatically mean sex.