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The fastest least expensive option is for the parents to give you temporary guardianship. Which is signed and notarized by the parties. This will allow enrollment in school and handling of medical issues. The only drawback to the guardianship is that the parents can revoke it at any time. If you file for custody and obtain a court order the parents cannot change it without having to file for a modification in court that would be granted by the judge.
The customer is obviously more stable but also more expensive and time consuming.