Aww bless him.
I get them really bad when I have a cold and they are so painful.
Other things you could look at are nervous habit kind of things like chewing or sucking or lipping his lips etc... his teacher may have spotted something he doesn't do at home.
Some kind of allergy or irritation is also possible, my mouth gets really irritated when I have certain fruits and nuts.
Keeping an eye on anything he might be sharing too.
Hope he finds something to sort it. When I have had them before I found chapsticks and stuff were little use and ended up asking in Boots and one of the girls recommended this...
http://www.cymex.co.uk/
Great stuff but not sure if it's ok for children though.