Lunch-time lock-ins.

According to the news, schools are being encouraged to keep their pupils in at lunchtime to avoid them going down the local burger-bar or chip-shop. Apparently some schools are already doing this. My question - to which as yet I have found no definitive answer - is, can they legally do this? Doesn't this amount to false imprisonment? If I want my son/daughter to come home for lunch, has the school the legal authority to stop them? What this seems to come down to is, just what constitutes the "school-day" - does it include the lunch-break? A quick Google suggests that different schools have different rules - some specify that you must go home for lunch (presumably these are those with no canteen facilities), and the vast majority seem to make it an option. Anybody know just what the legal position is?