Separation anxiety

My LO is soon to be 22months. He have been crying when we drop him off for school Everyday. he have been in playgroup for 4months already. But teacher told us that once he is inside he is not crying already. What can we do to don’t let him cry when we bid goodbye. I will always tel him we will pick him up at 6pm, school have lots of fren and things to learn. But he still will cry the moment he saw the teacher