In some ways, it's actually better that their outdoors rather than locked up.
You're more likely to catch influenza in school, because influenza is aerosolised and any close proximity means you're likely to breathe it in.
The coronavirus isn't aerosolised anywhere as easily. The reason that NHS has advised washing hands so strongly is that C19 is spread primarily through droplets that have landed on surfaces which people then touch and bring to their own mouths/noses. It's a physical route rather than an airborne route.
You've got more chance of catching C19 from a family member cooped up at home, than you are from walking around town or sat in a classroom practising social distancing and decent handwashing. This has been borne out in the statistics (75-80% of the spread has been pinpointed as happening in the home rather than in public places).