Some early clues of childhood OCD include changes in their routine and behaviour, including doing something over and over and becoming intense and withdrawn.
I cover this in more depth in my blog: https://themindmonsters.co.uk/news-%26-blog/f/childhood-ocd-how-to-spot-the-signs-before-it%E2%80%99s-too-late
Possible causes of childhood OCD include moving home or schools, fear of becoming isolated from friends and bereavement.
I cover more examples of OCD triggers in my blog: https://themindmonsters.co.uk/news-%26-blog/f/5-possible-causes-of-ocd-to-look-out-for-in-teenagers
If your teen is avoiding school due to OCD, try finding ways outside of the box to help them, including using online resources and working from calm locations.
I cover more tips on supporting a teenager with OCD with schoolwork in my blog: https://themindmonsters.co.uk/news-%26-blog/f/supporting-a-teenager-with-contamination-ocd-get-into-university