My step son is having a really hard time at school, and his mother isn't doing anything should I take over?

I am the stepmom of 4 children (each child has a different mother, but same father, 3 live with me full time, the oldest is 12 & severely handicapped, an 8yo and my 7mo baby-- so lots to do!!The other two 10yo and 3yo I have half time). Two of them 10 & 8, and are diagnosed with ADD and ODD.Our 10 yo is having difficulty in school, he has been kicked out of the last two daycares and is "on contract" at school for 6 offenses this month!It is obvious that something is going on, possibly with his medication not being at the right dosage but he is with his mother during the week, and she doesn't feel the need to do anything about it, just more punishment.Our 8yo had the same problems but not since he moved in with us.We are very structured, but loving & I make time to handle his situations so now he is doing very well!My problem is do I try and take the 10yo from his mother in order to help him do better and succeed (more work for me)or just pester his mother to do something about it?