Toddler sudden change of mood

Recently my toddler once steps inside the school will start crying compared to earlier days he will walk in, smile and wave at teacher. I suspect if he has bad experience with a person in school or other factor? Is there any ways to solve it? I have checked with form teacher, just simply said caused of environment change. I don't believe this is the caused so that why seek for people who got this experience before. ##Needadvice