Being a priest is a position of trust, just as being a doctor, a lawyer, or a professor is a position of trust. But if I begin a sexual relationship with my doctor, lawyer, or professor I do not expect that to be reported to the practice manager, the senior partner, or the vice-chancellor. It may be unethical, but members of the public do not need to police ethics based on nothing more than suspicion. If the priest was having a homosexual relationship that was wrong, just as it would be wrong if a priest were having a heterosexual relationship, which could be more serious, as there is the risk that the woman will become pregnant, which opens up all sorts of new possibilities for people getting hurt.
The issues seem to be (a) how compelling is the evidence? and (b) how closely involved is the person lodging the complaint? If I caught my own parish priest in flagrante delicto with a sexual partner or either sex I suppose I’d consider it not unreasonable to tell the bishop, as my evidence would be certain and the impact on the parish would be clear. But in this case the OP has no particular relationship with the priest and his only evidence is suspicion. If I thought that five years ago I may have seen a priest slip his arm around a woman’s waist in the supermarket I would not consider that it was my place to police his morals by reporting him to the bishop. For one thing, a perfectly innocent explanation would be possible.
You are still overlooking the basic fact that the precise reason for the OP’s post here is that the priest has now got a job at a high school. If the priest were now working for the curia in Rome or working as a chaplain in a prison or the armed forces the OP would not be posting here. The reason he is posting here is because he is worried that the priest is a danger to children: ‘Right now he is a priest assigned to a local catholic school which is 8-12 grade. … given the news recently, I should probably let someone know, right?’ All I am trying to assure the OP of is that the possibility that this priest may or may not have had a homosexual relationship with an adult man some time ago has no bearing whatsoever on his safety working with children in a school. There is no evidence at all that homosexuals are more likely to abuse children.
In the final analysis, what this comes down to is nothing to do with child protection, since there is no reason to think that the man is a risk. The issue is basically, if we suspect that a priest may have had a homosexual relationship, do we have to tell the bishop? Sure, if you have solid evidence of an ongoing or recent relationship that is causing pastoral issues, go ahead and report it. But if it’s a mere suspicion of something that may or may not have happened years ago and doesn’t seem to have impacted anyone, it’s not our place to go potentially ruining people’s lives. If the priest is corrupting people and causing a scandal somebody in his new ministry will be sure to highlight this to the bishop. If it doesn’t involve criminality or a risk to vulnerable people I for one do not want to become the parish policeman reporting people’s sins to the authorities.
