Keep your eye on the situation. There are too many what ifs with any day care situation. Do pop ins, talk to him regularly about his day, and don't discount any unusual scratch as a sign that he's just having an active day or just being a kid.
Sorry to be a downer, but bottom line it's hard being a parent who has to trust others to watch their child during the day. The minute you let down your guard or ignore your gut is when bad things can happen.
I am from the school of thought that too many minor scratches and bruises might mean your daycare providers aren't being attentive enough and are either leaving the kids alone too much and they're getting into stuff or situations they shouldn't be in, or the providers aren't doing enough to prevent toddler squabbles or just don't know how to handle toddler behaviors if squabbles break out. Both are potentially dangerous situations for your child if left unchecked. Neither are excusable in my opinion and would be justifiable cause to find a new daycare situation. I think it ironic that this would be happening in a "Montessori" school as the whole philosophy behind this is curriculum based and therefore the children would have few opportunities to get hurt or be left unattended where something like this would happen. Is it a Montessori school in name or by practice? Important question to ask yourself I say.
If the bruises are real serious and the teachers always have a convenient answer for why or your child starts acting out at home or having tantrums when it's time to head to school, time to start looking for a new center. JMO