Children so admitted shall be educated and maintained in the Scotland School for Veterans' Children until they shall severally become nineteen years of age, unless sooner discharged for cause by order of the board: Provided, That the board may at its discretion extend the time of the discharge of any child until the end of the school year during which such child reaches the age of nineteen years. For the purposes of this proviso the school year shall be deemed to commence the first day of September of each year.