If the teacher is not an attorney, or has not had an attorney draft the contact, the language may very well be flawed, as you propose.
From a practical standpoint, you would almost certainly need to engage an attorney to challenge the contract in court, which would be expensive, unless they were willing to work pro-bono.
The wording of contracts can be critical, so your instincts are quite good regarding this angle of attack.
However, based on the sample you provide, my understanding is that the parents are consenting to the teacher collecting and holding the phones overnight.
From a constitutional standpoint, it initially depends on a key factor:
- What are the laws in your region in relation to this issue?
Kids are generally afforded different rights than adults, ideally more protections, but also more restrictions.
I did some googling on this subject but have yet to find anything relevant. (Most calls for cell phone bans in Europe seem to relate to the idea that extreme overuse might cause cancer: http://www.reuters.com/article/us-italy-phones-idUSBRE89I0V320121019 My understanding is that there is still a great deal of debate as to whether this is so, and can serve to illustrate the interaction of the law and science.)
- If there is no legislation regarding the rights of minors to carry cell phones, or be restricted from doing so, have there been any court decisions related to the subject?
- If there are laws that support the confiscation of cell phone from minors where parental consent is given, is it possible the law is unconstitutional?
An activist might use the trip--getting barred for refusing to relinquish their phone, or because their parents refused to sign the contract--to generate publicity on the potential unconstitutionality, and reach out to activist lawyers willing to work pro-bono on the issue. (I could also see approaching cell phone manufacturers to back the suit for financial or PR purposes, i.e. fighting for the rights of kids to always be connected.)
I caution you that such suits, although always interesting, are generally extremely expensive, and can even be financially ruinous. The process, especially when dealing with constitutional issues, can take years and even decades. (Outcome of trials can never be known, thus settlement is generally seen as the most prudent choice in business litigation, although your issue is in a different domain.)
The first steps in challenging the constitutionality of this contract would be to:
- Research the laws and statutes of your region, which are probably all available on-line
- Scour law reviews for articles on this subject
- Search for articles from vetted journalistic sites on this subject (i.e. don't believe everything you read on the internet;)
From a personal standpoint, you will want to weight the cost/benefit of challenging your teacher's contract:
- Is what you would gain worth the sacrifice to attain it? Is it worth the risk?
Justice and the law are not always synonymous, and even if your cause is righteous, you may not prevail. Even prevailing, you might find the cost was too high.