Taking down religious symbolism does not prohibit the free exercise of your religion. This is public property we're talking about, except for the case of private schools. If you want to open a private school and hang all the religious symbols that you want, you are certainly free to. But when its public property being paid for by public tax money, religious symbols do not belong there.
No, its completely within accordance with our first amendment. You have freedom of religion, but you do not have the freedom to push your beliefs on others. Schools are public property paid for by our tax dollars (except for in cases of private schools) Religious symbolism of ANY kind has no place on our public property. It's not oppressing your religion when I prevent you from pushing it down other people's throats.