Under the federal law called The McKinney-Vento Homeless Assistance Act (42 U.S.C. § 11431-11435) this young person may be considered an "unaccompanied minor" and is entitled to continue to go to school in the same high school, even if the teen does not live with the mother. This law requires school districts to provide homeless students access to the same free, appropriate public education. School will not be a problem. Even if this young person does not want to go to the same school, they will be assisted by the school district where they find shelter. The school has to be informed of the child's status.
The challenge will be to find shelter and there are many agencies that will help. Here is a website that provides resources and a crisis hotline for youth: http://calyouth.org/ca-youth-crisis-line/. Here is the phone number: 1-***-***-****. I suggest that this young person call and seek guidance for shelter and school services. If the young person is already staying with someone, this hotline can help give guidance about this as well.
It is possible that a person helping a teenager to runaway can be charged with the following law:
California Penal Law 272. (a) (1) Every person who commits any act or omits the performance of any duty, which act or omission causes or tends to cause or encourage any person under the age of 18 years to come within the provisions of Section 300, 601, or 602 of the Welfare and Institutions Code or which act or omission contributes thereto, or any person who, by any act or omission, or by threats, commands, or persuasion, induces or endeavors to induce any person under the age of 18 years or any ward or dependent child of the juvenile court to fail or refuse to conform to a lawful order of the juvenile court, or to do or to perform any act or to follow any course of conduct or to so live as would cause or manifestly tend to cause that person to become or to remain a person within the provisions of Section 300, 601, or 602 of the Welfare and Institutions Code, is guilty of a misdemeanor and upon conviction thereof shall be punished by a fine not exceeding two thousand five hundred dollars ($2,500), or by imprisonment in the county jail for not more than one year, or by both fine and imprisonment in a county jail, or may be released on probation for a period not exceeding five years.
(2) For purposes of this subdivision, a parent or legal guardian to any person under the age of 18 years shall have the duty to exercise reasonable care, supervision, protection, and control over their minor child.
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