Give it time - it took our little one about two weeks to settle into the routine. Both when he started and again when he changed from the baby room to the toddler room (which was not helped by him being of ill for a week right before the switch). He is still occasionally a bit resistant to go... but also a lot of the time he doesn't want to leave when we go to pick him up as he is having too much fun and playing with his friends.
When we first started, we did few play dates with the other kids in her class. She’s now really close to the other kids in her class, and wants to go to school so she can see and play with her friends.
Something so sweet and simple that I got from a sleep trainer is using the same short reassuring message with your little one at bedtime and drop-off. Ex: our bedtime words are "I love you. You're safe, mommy/daddy will always come back." She shares the sweetest story where her little boy got a few yrs older and thought he lost her in Target but she could see him, when she walked up to him, he stopped crying and said "mommy always comes back!" Maybe this can help over time, especially hearing you say "Mommy comes back!" During pickup. Hope this helps in some way!