Who do I hold accountable for my 5 yr old son being molested at school, by a child of the same age?

My son was instructed by his music teacher to go turn the light on in the bathroom for another student, When my son went into the bathroom the boy asked him if he wanted to "make sex with him?" The child proceeded to pull his and my son''s pants down and made their penis''s touch. The teacher was made aware of this incident immediately by my child and not until a day later did I even recieve a phone call from the assistant Principal. Only after my son''s father, 2 days later went to the school, did they remove my son from this class and kick the other child out of school. My concern is now for my child, who is now confused about what happened. but also I''m also very disturbed by the fact that there was a lack of supervision by the teaching staff. I would like to know what my options are to recieve justice for my child.