I'm sorry that she (and you) are having trouble here! Please, please rest assured that this is VERY normal even for kids who are otherwise outgoing and social and who usually like school. Remember, she has had only two Mondays to get through and is still probably in the post-summer letdown phase; weekends remind her of what summer was like (I'm home, I'm having fun, my time isn't all spoken for) and Mondays come as a bit of a shock to the system (my daughter is 12, knows fully what to expect, and still has a rough first few Mondays when school starts!).
You do need to talk to her about how she's six, and first grade is her "job" and she should be proud of that. Be sure to acknowledge her emotions and let her talk about them but at a time when she's calm and not in the throes of impending departure from you (Monday breakfast is not the time for that discussion, for instance). Ask what SHE thinks could make dropoff easier -- other than just not going to school. Let her see you listening.
You did not mention the logistics of how dropoff works at your school or for the two of you. Are you walking her to the school? Driving her and she gets directly out of the car? Driving her but parking at school and walking in from the parking lot? It does make a difference. As someone else noted, you must be swift and sure in dropoff. Do not linger, give extra hugs or keep turning and waving at her; it may sound cold, but you really do have to make the parting fast and confident on your part so it sends her a message that it's time to get down to business and mom is confident she'll be fine. If you walk up with her to the school door, do not go inside; let her go in the door by herself (don't proceed into the lobby or to the classroom door -- just don't, and most schools dont' allow that or dislike it even if it's allowed). Make a ritual that is the same every time -- maybe one hug and kiss, a special saying you and she alone use ("Have a great day, you'll be A-OK" or something, let her make it up like a magic spell)and then you turn and walk away without looking back. If you drop her off in a car, start using the dropoff line for cars if your school has one, so she must get out of the car but you stay in it and drive away.
Whichever you do -- pick one dropoff technique and stick with it but ensure it's swift and you do not go back to see if she's OK, or follow her to the classroom as the teacher escorts her, etc.
If she is still having these issues after maybe the first five or six weeks, I would ask to see the counselor -- you at first, without your child -- and ask the counselor for some strategies and ways to handle this. If she really persists, ask the counselor to meet with her one on one a few times to see if there are other stressors, but this does not sound like there are, if she's fine once you are gone. Really, this will pass.