My son and school :(

So this my son's first week in school. He's 3 years and a half. he was fine the first two days and today he hit one of the children and he started throwing toys on them. When I went to pick him in the afternoon his teacher told me. I got really upset of his behaviour. He's not like that at all. I started crying the minute we came home. I don't know what to do ? I'm scared he will do it again :(. Please feel free to give me any kind of advice. Thanks