Andover Public Schools remain closed as hundreds of teachers continued to strike for a fourth day. The school and the teachers’ union have failed to agree on a new contract over the weekend. Classes have been canceled for students since a Thursday night vote by the Andover Education Association to go on strike. The school committee says it could not agree to a union proposal of an 18% raise over four years for all teachers. The committee says that cannot happen without the town overriding Proposition 2½ at a townwide election. There are 5,500 students in the school district.