Section 25A. The assessors may appoint as assistant assessors such number of suitable citizens as they deem necessary, and may remove them. Such assistant assessors shall perform such duties as the assessors may require for such time as they may determine and shall receive compensation only for duties performed as so required. Every such assistant assessor shall hold office for the period of one year from the date of his appointment, unless sooner removed; provided, that, in the case the assessors fail to appoint a successor to any such assistant assessor during the month preceding the expiration of his term, he shall continue to serve during the following period of one year, unless sooner removed. This section shall apply in all cities and towns, except Boston, notwithstanding any provision of special law.