Am I overreacting? Teens in my yard

So, we have had an issue with neighborhood boys cutting through what is very clearly our yard after they get out of school. We own 4 connected lots in town and they don't cut through just a portion of it, they cut across the entire yard and walk right past my living room window inbetween my house and garage. We don't know them or their parents, we've only found out about it because I've been finding things in the yard that don't belong to us (glasses,notebooks, pop bottles etc) and I recently became a sahm so I'm home when they do it now. I'm pissed off about it because we don't know them, they leave stuff behind, I feel its an invasion of my privacy and I don't want to be liable if my dog bites one of them (shes a sweetheart but freaks out whenever she sees them in the yard so I have to keep her in around that time.) Id probably be okay with them doing it if they asked. Am I justified in being upset or should I just forget about it?