My son rubs his penis on the edge of a table or bed and he is doing this in school also this distracting his studies also

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Have the teachers mentioned this to you?

First thing I would do is have him in to see the famlily Doctor to make sure he doesn't have some skin irritation, or allergic reaction thing going on. Some fabrics, and fabric softeners cause discomfort and itching.

If there is no medical cause, then you need to teach him, that it is inappropriate to rub himself on his desk, or bed, or anywhere. If this is unchecked and not corrected, a teacher will notice him next rubbing up on another kids' desk, and will have to take his behaviour more seriously.

My guess is it is a habit that needs to be stopped. He's only 7 after all.

Sounds to me that he's reaching puberty, and this is his first venture into self pleasure. It may be young but it does happen at that age.

Otherwise I agree with Jake, take him to the doctor and make sure there's nothing wrong down there.