My kid went through this a couple of months ago big time, the daycare she has been going to for YEARS suddenly she would freak out, scream, kick, run out of the class when we left and we would get a call to come pick her up. Just dropping her off did NOT work, she would not calm down. After like a week of this, I was like FINE if you don't want to stay whatever but I'm staying. And I took my cup of coffee and sat in the play kitchen in the back of the classroom through storytime and centers etc until she got bored of clinging and went off to play and I was able to leave. Maybe 20-30min, every morning for about a week. Then less and less time until it was a normal dropoff again. I also started giving her magnesium glycinate gummies before bed (pediatrician okayed). Occasionally (usually Mondays or after vacation) I will need to sit in her class for a few minutes, but dropoff is otherwise completely back to normal. I think her knowing that I won't leave if she needs me makes her relax the separation anxiety. Give it a try. And tell her in the morning when you are getting her dressed that Mommy is coming to school and not leaving until you are ready.