You cannot be compelled to sign a form indicating that you agree to something. However, your lack of agreement does not override a policy that they have authority to set. There is a contractual way that this could work out for them, depending on what exactly the document is.
To be a contract, the parties must agree to the terms voluntarily, and if you do not agree to the terms, there is no contract. A 10 year old child cannot be bound to a contract, anyhow, so the child's consent is legally irrelevant, though strategically a good idea in the sense of alerting the child to their obligation. To be a contract, both sides must offer something that they are not already obligated to provide. What is the school offering?
On the school's side, they might claim "We offer an education", but as a public school, they already have that obligation. Schools have broad authority to impose rules in order to operate, so in lieu of a successful lawsuit that the district overstepped their authority and violated someone's constitutional rights, the school could have a policy prohibiting use of a cell phone in school. Paired with such a policy, they can grant conditional permission, subject to the parent (and symbolically, the child) agreeing to certain terms. Since they are not obligated to allow cell phones at all, they are offering something of value to you, and you have a contract. The cell phone owner could try suing the school for keeping the phone, but the suit would fail because there was a breach of the contract. A strategy probably not worth pursuing is arguing that the confiscation clause is unconscionable (which would void the contract, which entitles the child to have a cell phone at school).
Confiscating the phone is not theft, since the intent is not to permanently deprive the owner of their property (just as it is not theft when you have to leave guns or recording devices at the security desk). If a student were to take a forbidden thing without the owner's authorization (such as a gun, or a phone) and it was then confiscated, the rightful owner might be able to sue the school – as long as their hands are clear (if they had no knowledge that the thing was taken and used in an unauthorized manner). In this case, the parent clearly knows and authorizes.
