You've got a load of parents who are returning to work over the next few days, leaving teens alone to do school work.
Some will follow rules, even though they do realise it's not any benefit to them and others won't. There's not point lying to a bright 15 year old that they are staying in to keep them safe, it's pretty clear to any who are able to access the news media that they are staying at home to keep other people safe as it's about as likely to kill them than the flu, and you don't keep them off school when there's a case of the flu.
In the same way you could argue that a 15 year old should just be told not to have sex, many parents would rather say, if you are going to, this is how you do it safely.
You'll probably find that some parents have taken the view that if they can't keep their kids indoors and can't be sure they aren't going to meet up with friends/ boyfriends/girlfriends, then it's better to have a chat about how to do it safely - outdoors, staying at least 2 metres away, more if downwind. Not touching anything the others have touched, so eg it's not ok to share a packet of crisps if one person puts it down then steps away while the other one takes a handful. Ask them to wash hands and change clothes when they come back indoors.
15 year olds shouldn't be smoking, drinking or having sex. It's against the rules for everyone to take drugs. Lots don't. Many do. Lots of those who do, have parents who think they don't. Let's discuss risks and how to do things as safely as possible.