Just to say that yes, schools write that pupils should not be in a public place during school hours, however it is not actually illegal for them to be in a public place at any time as they may have good cause, IYSWIM, so it makes it difficult if not impossible to enforce. There is no obligation on the part of the school to make childcare or alternative educational arrangements.
However a solution for schools if parents are not being supportive of the exclusion is to do it internally in school, i.e. make the troublemaker sit in a room with 1:1 supervision and do extra work, staying away from other pupils at all times. Of course, schools need extra manpower for this so many of them won't do it.
It is a problem with no obvious solution, short of insisting on the 1:1 thing actually in school, via legislation of some kind, and putting the onus on schools. Maybe we will see this in the new Children's Act.
Engaging with errant teens if you feel you can, and persuading them school is a good place to be and everybody cares how well they do there, is a very good societal measure, and I would urge anyone who comes across a local teenager in this situation, whose behaviour leaves a lot to be desired, to be civil, pleasant, and show an interest, engaging them in conversation to this end over a period of time, so relationships are built. It won't bring instant results, but the cumulative effect of them being treated as worthy of this kind of attention often takes the edge off some of their nonsense. People are usually afraid of doing this in case they are duffed up or whatever, which is half of the problem, but really this is no more likely to happen than a duffing up as a result of a road rage incident on a hot summer's day, i.e. probably a one in ten thousand chance. The worst that is likely to happen is that they tell you to fuck off, which is hardly a big deal and you can always say "OK then, see you around" and do so. 