Any teacher who has been credited under the rules and regulations of such board of education with an aggregate of thirty years of teaching experience may be retired by such board of education. Any teacher so retired under the foregoing provisions of this section, provided that at least twenty years of such accredited teaching experience shall have been in the public schools of such cities of the first class, shall be entitled to receive from such retirement fund so long as such teacher may live, equal monthly payments which shall aggregate five hundred dollars per annum: Provided, however, That no one shall receive such pension without paying into the fund therefor, by way of assessment or otherwise not less than the amount of the first annual pension to which such person shall be entitled. And in order to make up such an amount the board of education may provide for any deficiency by deducting the necessary amount from the first year’s pension payments in equal amounts each month.