In the past year, my son has been going to the toilet more often than usual, sometimes every 30 minutes. Other days he just goes 4-5 times in the whole day. Doctors checked his urine and blood and everything seems to be normal so we stopped worrying long ago.
In the past 2 weeks this has been happening almost every day, he feels fine, happy, eats well, etc... but since he arrives from school until he goes to bed, he uses the toilet at least once every hour.
A colleague of mine says that at this age hormones can make teenagers' bladders more active and that it's normal so I wonder if this also happened to some of your kids too. I remember I had the same problem when I was 15 and then it just disappeared...