Should students be allowed to wear "satanic" symbols to school?
I think schools should allow for a lot of self expression, including forms of religious expression, within some reasonably defined lines that serve the school's purpose while allowing the students the most amount of freedom possible in this context. This means that naturally satanic symbols should be allowed in my view.
At the same time though, i think that doesn't translate into their being no possible exceptions generally, and more importantly, i think that any line set might eventually contain some element of seemingly unnecessary exclusion or limit, at least from some points of view. The school's vision affects where the line will begin and end, and it might never be suitable for everyone, but some lines will be far more reasonable, inclusive and effective than others.
Is a cross necklace, or any of the symbols being spoken about obligations?
As for things mandatory, id say deserve some lax but not total. Hijabs should be kept to minimal, no niqab, and must meet other proper dress codes. Same for kippahs and the headress sikhs wear
Then perhaps it's more reasonable to have a general rule for allowing some basic forms of religious expression that can cover both such obligatory things as well as some basic things other religious followers might like to wear, so long as it doesn't go against the school's purpose.
Like let's say a person decide they are a worshipper of Priapus and to them that means they have to wear a huge dildo on their heads...
A 'huge dildo on their head' isn't any form of simple jewelry or an article of clothing, which is different to what a school might be reasonably trying to allow. It's a sex toy, so it's not something that's normally worn or taken to public places, and so it's not the same thing. In this case, the religious expression in itself is not what's not allowed, rather the specific method. The same person could be allowed to express their beliefs differently, just not in particular methods that might be outside of what a school is making an allowance for.
It's a poor example to try and make your argument with because it's unrealistic and more importantly, doesn't result in inconsistency if not allowed in the hypothetical. A general principle of allowance for religious expression can be present, but that doesn't mean that the allowance can not contain further defining factors to make it more precise and clear.