By "kept behind", I take it you mean detention, no school bus schedule is so tight that it leaves within five minutes of a school day ending.
A summary of the law with regard to detention is given in the DfES circular 10/99 Annex C - Detention: the law and how to apply it:
If the detention class is arranged for after school the parent must, by law, be given at least 24 hours advance notice by means of a special form for the parent to sign and return. The notification to the parents is to inform them of the detention and is not intended as a request for permission. If the signed form/letter is not returned the detention should still be valid but it is desirable that the parent should be informed.
So basically, if you (or the pupil) knew about it, you should have made alternative arrangements, if the pupil didn't tell you, that doesn't affect the school, they followed procedure, if it was an immediate detention (kept beyond the usual school day) at the end of a class, then the school acted unlawfully and you are entitled to be compensated for out of pocket expenses.
But....for £2.20, is it worth it, just phone the school, tell them that they were in the wrong, quote the DfES guidelines and ask that they remind all teachers about it. Play nice, something might get done, go in guns blazing.......
DfES Rules Regarding Detention 24 rules still in law (read the small print at the bottom)