Is there a specific reason you don't want to walk her to class? For instance -- you have a younger child in the car and it's a problem to park and get that child out and haul him or her along? Or you are on the way to work and it takes too long to park and walk your child into the building and it makes you late some days? Or is the issue none of the above and you mostly just want her to exercise the independence you saw at the start of the year? It would help to know the full circumstances.
If you have another child to handle at the same time, or you are headed for work and really need her to get to class on her own from the dropoff line, then you and she will have to work on this. Did something happen around Christmas? Did the long break at home perhaps make her more clingy? (If so -- give it a little more time or cut a deal that you walk her in only on Fridays because it's a "special day" or whatever it takes to wean her off having you walk her.) Or did something happen in class itself that has made her need the reassurance of having you around a little longer in the morning? I'd ask the teacher. Very small things that seem insignificant to an adult can really throw off a young child and make her want mom.
If your frustration is solely because she was doing it for a while and now is not -- if you do not need to deal with another child, or get to another child's school dropoff on time, or get to an office -- consider just continuing to walk her in for a while. In other words, if there is not somewhere you have to be immediately after dropoff, why not walk her in for a bit longer? She needs it; it's her first real school experience; and all too soon she will reject any offers to walk with her into school or anywhere else. You can gradually cut down how far you'll take her: For a few days, you go all the way to the classroom, then tell her you will say goodbye at the end of her classroom's hallway and watch her walk the rest of the way down the hall; then you eventually tell her you'll go as far as the school lobby and watch her walk down the hallway leading toward her wing; then you only go as far as the front doors and not into the lobby, and so on. Unless there is some real and pressing need to get away from school fast, it cannot hurt to walk her in and gradually go shorter and shorter distances into the school.
And if you have time, being able to hang around a few minutes after dropoff is a good time to talk to other parents who also walk kids to the door. It's how I met some good friends and also how I learned a lot more about the school than if I'd left immediately each day.
Of course if there is a real need to have her exit the car and you don't have any time for anything else, you may need to work with a reward system to get her to do this, and maybe even do some practice runs when the school is closed. Yes, she's done it before, but she might find it fun to do practice runs....
