Did you have a written accommodation plan or was the egg free classroom a verbal agreement?
I would do two things--
1) Talk to them about how you feel about what happened and make it very clear that you want and expect them to follow the agreement that the classroom will be egg-free. Tell them it is NOT OK to be sending your child out of the classroom during the school day. It is a complete waste of instructional time and it is excluding your child and it is potentially putting your child into danger if food residue is left behind. I would get a note from your allergist or doctor stating that your child's classroom should be allergy-free to prevent reactions to back you up.
2) Write to the 504 coordinator of your child's school and request a 504. I recommend reading sample 504 plans here and elsewhere on the internet. Then write a sample requested plan you want for your child. (If you already have an Individual Health Care Plan in writing you can have the 504 plan say, 'see IHCP'.) Then when you meet with the school you can use your sample requested plan as a started point for talking to them. They may refuse some accommodations but you can negotiate back and forth until you are satisfied with the plan.
The fact that the principal knew about and allowed this to occur isn't good, though. It shows a lack of sensitivity on the part of the principal.
Did you call the ADA? What did they say to you?
Having a 504 does not guarantee you will get what you want. All it does is give you legal protection if your child's plan is broken as your child's plan was. It would still be good for you to get one. You can print out info here and from various food allergy websites proving that food allergies qualify for a 504. I can send you a PM that has the argument I made with my son's school that food allergies do qualify and it worked well. Before I read my statement to them they didn't think food allergies would qualify and the principal had told the nurse they do not. After I read my statement there was silence and then the head of 504s said, "well, I think we can all agree that food allergies clearly qualify" and everyone said, "yes".