WASHINGTON — The Trump administration is weighing a new policy that would fast-track the deportation of thousands of Central American teenagers who arrived at the southern border, unaccompanied by adults. According to two sources with knowledge of the developing policy proposal, the Justice Department under Attorney General Jeff Sessions wants to avoid creating a new protected class of undocumented immigrants, given how politically difficult it has been for the administration to unwind another program — DACA, which protected young people brought by adults to the country illegally when they were children. This new policy would call for expedited deportation of another group — the more than 150,000 children who arrived at the southern border alone, escaping violence and poverty in El Salvador, Honduras and Guatemala.