I would side with @Lilienthal's sentiment that being moved off front-desk duties does not at all seem like a step backwards, on the opposite being relieved of that type of secretarial work seems like a reason to celebrate. Think about how much time that ate up.
You now have that time to work on your professional development, building skills associated with more integral roles closer to the heart of what the school does -- classroom practice.
Think about it this way: what amount of training, past experience, and credentials would someone off the street need to do each particular role that you did. What does it take to be a classroom monitor? What does it take to be a front-desk staff? (I would guess these jobs can be done well with a HS diploma, good work ethic, communication skills, decent memory for names, and a few months on the job to learn the ins and outs of the building and the school year cycle.) As a result, an individual in these roles is highly expendable and can be flexibly shoved across these roles without much ripple effect productivity-wise.
So I would suggest to reconsider your situation. Where there appears to be a downside, there may be a great opportunity for growth. What could you do with the time and mental capacity freed up from the meet-and-greet duties, that would move you closer toward the goal of becoming a valued expert in some narrow area(s) in the public education system? Perhaps you could take on additional roles that your experience as classroom monitor affords, such as participation in internal curriculum development or teacher peer evaluation efforts. Perhaps you could become more involved in supporting peripheral activities, such as administration and coordination of assessment activities, instructional coaching, technology or library services?
Again, I think your year is off to a promising start -- the difference between whether it will be just another year of same old, or a year characterized by a leap forward professionally, is a matter of perspective, wise goal setting, and perseverance.
Look into PD opportunities at the school and district level, think about where you want to go professionally, find alignment and pursue these. Now, that might indeed be worth discussing with your principal. Come to him/her not with an attitude of a (slightly) ticked-off employee, but that of a mentee -- "I think I am ready for new challenges, I am interested in growing professionally in areas X, Y and Z, do you think there are opportunities you could recommend or additional support roles you could see me succeeding in within these areas?" And so forth. Everyone likes to be a mentor, sometimes you just need to go out on a limb to put someone in that position. Often, that's all it takes for them to reach out and provide the support we seek. Good luck!
I also worked at the front desk greeting parents, working security and doing some clerical work.Did you also lose these tasks in addition to the front desk? Did you do all things at the same time, or were you moved from one position to another over periods of time? It sounds like you're left with monitoring 3 classes at the moment, having been reassigned from the positions mentioned.