As in: My phone was just confiscated, but not before I shut it off. If they wanted to look through it without a warrent, can they force me to decrypt it?
EDIT: USA is country-of-residency
Public schools are on a shorter legal leash than private schools are, because they must behave like proper governments do and respect the constitutional rights of their charges. (First Amendment rights are much broader in public schools than they are in private schools). Assuming that we're in a public school, a search of your phone is governed by a watered-down version of the 4th Amendment. They may search your phone if they have a reasonable suspicion that there is evidence of a violation of the law or a school rule, but this is passive with respect to you providing information -- if the phone is on, they can snoop around if they have a reasonable suspicion.
The 5th Amendment would be relevant to passcodes: they cannot compel you to give up your passcode. If there was plainly-visible evidence of wrongful activity on the phone which they saw, and then you shut the phone down, then analogous to in re Boucher the courts could order you to reveal your password, under the "foregone conclusion" doctrine. However, it's the courts and not the schools who get to make that determination.
One way the school could literally force you, bypassing the legal system, would be to physically threaten you, by beating you or threatening you with a gun or whip. Such physical coersion would be a felony, and it is almost inconceivable that they would do that. What they might maybe do is give you a non-physical ultimatum, of the type "decrypt the phone / reveal the password or we will... expel you / fine you / fail you in your English class / not let you go to any more football games / turn the phone over to the police". The question is, what would be legal versus illegal by way of consequences?
I know of no constitutional right to attend football games anywhere, so they might well be able to get away with that deprivation. You might have a contractual right to attend games at a private school. With a private school, there is some contractual agreement between the school and you (least likely, assuming you're a minor) or your parents~guardians (most likely). That contract could imply certain rights, such as attending games, and might spell out a procedure for them to terminate that right. If they don't follow the process, they could be in breach of contract. Apart from contractual rights (which probably involve your parents, not you, but also would be considerable, for example the right to attend and be graded fairly in the English class), you have no protected rights.
As you can see, the answer has a lot of "it depends" in it. Suppose your public school had a reasonable suspcion that you had engaged in a criminal activity, and might prove that by looking at the phone – which you shut down. They cannot use physical force against you, but they can try to persuade you, by offering you something that you want which they can legally take away – like attending a football game. If they try to deprive you of something that you have a right to, you can sue them to prevent that. However, they can also seek a court order to compel you to reveal the password, and it is certainly not illegal to inform you that if you don't unlock the phone, they will seek a court order. Whether or not the courts will grant the request is not obvious (incidentally, if the device is fingerprint-protected, you are hosed, since forcing a person to prove fingerprints is not against the 5th). If the school can be very specific about having seen criminal evidence, they have a leg to stand on, otherwise you cannot be compelled to testify against yourself (coughing up a password is a form of testifying against yourself).
I don't think mine will be the best answer, but it's worth noting that case law states that children do not “shed their constitutional rights at the schoolhouse gate” from Tinker vs. Des Moines Indep.Community Sch. Dist., 393 U.S. 503, 506 (1969) - however that seems to be a 1st amendment issue.
As to the legality of searches in schools the Supreme Court states in New Jersey vs. TLO 469 U.S. 325 in 1985 held that the Fourth Amendment’s prohibition against unreasonable searches and seizures applies to school officials and is not limited to searches performed by law enforcement. It seems to say that school employees act as agents of the state when they perform their duties of the school.
It looks like the standard for a school employee to search is not probable cause, but rather if the search is 'reasonable' - which gives a wide range of areas open to interpretation. I would highly doubt they could force you to decrypt it. After all, you might have forgotten your password.