Syag: If there is unquestionable (videos, witnesses, admissions) evidence that a well respected man abused several children ch”v then almost certainly he should be reported. Remember, Al Pi Halacha reporting to secular authorities is to secure one goal — namely, stopping him from causing further abuse. Theoretically, if that could be accomplished another less severe way, without reporting, we would have to use the alternative. But in such cases it is unlikely to accomplish that goal with any other method. As such he is to be reported.
That all said, if we’re talking about a case where a (real) victim brings up a case of his/her teacher who (really) abused him/her 18 years ago as their teacher in school when they were a child, and today that teacher is a feeble old man who can’t leave his old age home without a wheelchair, then almost certainly it’ll be forbidden to report him.