Even leaving the cost aside, the prospect of success after many years is inevitably significantly less than it was when the evidence and witnesses’ memories were fresh.
And even leaving all that aside, there is a very real possibility that a judge might throw the case out as soon as the trial started on the grounds that, as the accused has been a man of exemplary character in the many years following the alleged incident, it serves no legitimate purpose to try the case now.
In practice, the boy’s best plan would be, immediately following the incident, to enlist the services of his father together with a couple of burly uncles and to have a private conference with the headmaster down some dark alley or other.