SzeWing,
How old is your girl? My girls is 27 months and have the exact same experience. When she returned to school in September, she had no problem. Wave bye bye to me and run to the toys. Then after 3 weeks, she all in a sudden cried in the middle of the day and began to refuse to go to school. It lasted about 1 week or 2. During the period, I stayed calm and talked a lot about the positive things in school. When I dropped her off to school in the morning, I will give her a big kiss and wave bye bye to her cheerfully. She may be crying but that's ok.
Now, she returns to normal and no fuss at all.
Once they realize they have no choice and mommy always come back, they will be fine. It will be easier if they have a special teacher whom they like a lot.