1) This is common for this age and just starting Kinder.
My daughter was similar...
2) Is this a public or private school? My daughter's school is public for example, and for the Kinder kids they do this: at the beginning of school, ALL the Teachers' are extra observant for any Kinder kid that is having transition problems. IF a child is... then, there is a Counselor that will work with the child. My daughter had this. All it is, is a 1/2 hour session with the Counselor, for 1 month... they do "play" type counseling, letting the kid vent, to say their feelings, to feel 'safe' that the Counselor or SOMEONE is there to help them and that the child can go to this Counselor ANY time, for times like this. AND the child then has a "Buddy" to go to, for any reason/help. This approach, per my daughter's school, helps IMMENSELY.
2) Do not admonish him for his feelings. It is normal. A kid, NEEDS a place to 'vent' about his day for good or bad. That will form a LIFE LONG communication/relationship with your child as he gets older. Invaluable.
3) And any time a child is teased... TELL THE TEACHER. This is unacceptable. My daughter's Teachers, all of them, ALWAYS talks to their kids about proper behavior and "bullying." AND they will not/should not single out your kid.
4) Next, what the heck, does his Teacher DO when your child is crying/crying for you???? Because, THIS will impact how the other kids act toward him. If he is being teased on the bus... I would tell the Teacher. This is bullying.... and some kids even commit suicide over "teasing" and bullying. It is very real... and deeply affects a child. You as a parent, need to advocate/help your child.... in these times. Always.
He is already having a very negative experience at Kinder. That is sad. You need to address it with the Teacher.. ..and see what her attitude on it is. If you feel it is not appropriate, then well, that is unfortunate...
Does your son like his Teacher?
Sure it is transition issues, which is common in kids starting Kinder. But also, really 'hear' his concerns and see if his Teacher is good or not? Helpful/nurturing or not? Not all Teachers are Mary Poppins.
Sometimes it is just a maturity thing too.... is he really ready for Kinder?
And yes, it is a long day. But that is how elementary school is.
It is a mental and emotional adjustment... thus, toys or rewards or punishments will not work.
all the best,