The notion of the slips as I've seen them implemented is as such:
In X grade we will have education on Y topic which has some measure of content that parents may find questionable for their children. If you object to this then please advise us in writing (through whatever means the school prefers) and we will have them placed in an alternative class dring that time.
If the parent rejects the class then it will be presumed they will attend the class the next year/semester, if it is rejected again at that time then there needs to be some intervention to have a discussion to identify a specific cause. That could include school counselers, social services or whomever else is required to ensure the kids get a comprehensive education in accordance with the established modern standards.
Parental interests should be noted, but not the exclusive dictation of what information is available to a kid. Those who look to restrict access to information are almost universally going to be doing so because they're afraid that their own version of things is going to be seen as wrong, usually for good reason. Ignorance is bred in isolation through unopposed repetition of opinions stated as facts.
Helping shape curriculum is distinct from the choice to engage with it at any given point and not mutually exclusive. Arguing for the exclusion of information is almost always bad, arguing for the inclusion of it is less often bad but needs to be backed by sound logic and science.