I experienced this with my 2.5yo too! If nothing has changed in the school environment, (change of teachers, classmates, bullying etc), then she may just be missing you. Maybe you can try to spend more time 1-1 time with her on weekends or afterschool to build her emotional confidence level? What I did was, we skipped school for a thurs & fri and spent the long weekend doing a lot of activities and bonding, but at the same time reminding her we'll have to be back in school on the Monday. It did helped. She was less teary that Monday and it got better. Good luck!
Aww. Sorry that your little one is being emo. Is there anything about the school environment she does not enjoy?