tbh, it's just girls growing up. I know when i was around 11-15 it was all "ooo i fancy this boy" etc it's just all part of growing up, it may just be totally innocent. i.e girls scrawl all over their books etc "fi 4 ? true love always" etc etc but there's more often than not, NOTHING going on between said girl and boy.
Just give them the benifit of the doubt. I know its hard, and you're trying to protect them, but you really have to start trusting them. 13 is a difficult age anyway, dont have a go at them or anything cos at 13 you're already "
omg, i'm not talking to my DAD!" so you dont want to risk pushing them away over something silly.
If it were me i'd just do as said above and start a convo like "oooo, so who's james" just jokingly winding them up kinda thing, nowt serious. they'll prolly just say "boy in my class i fancy" then you can just relax
if they say "my bf" then
tbh, they prolly just have a quick peck on the lips after school
as i said, trust them, they'll thank you for it later