It is an invasion of privacy to seize phones without a good reason.
Like it or not, even pagers have been banned from schools. By and large policy is to keep them in the office until they are claimed. Don't think they can be "searched" without a warrant.
Not if you are an underaged student over whom the school acts in loco parentis. The school has a very compelling interest in mimimizing disruptions to education; high school students have no legitimate need to talk on the phone during school hours. The students have quite literally no reasonable expectation of privacy to keep their cell phones at school.