Kids often struggle with transitions and going to school is one of the most common.
Because they are having to shift modes and mindsets AND separate from us, the process of feeling willing to go and get ready can be emotional some days.
In my house Mondays are markedly the hardest.
Remember to hold the boundary, make sure there isn't an underlying need to be addressed, offer comfort and then ride the wave of emotion until the calm settles and you can get moving.
If you don't have enough time, you may have to make compromises (yes you can wear the dirty shirt, or eat cereal in the car etc) ,but stay compassionate even if you have to push to get to the next phase.