But nothing in this section shall prevent the retirement board from modifying the method of collecting the contribution of members so that employers may retain the amounts so deducted and have a corresponding amount deducted from the appropriation for the support of common schools otherwise payable to them.
The amount to be contributed by each employer except those who operate local district pension systems, shall be such percentage of the total compensation or salaries of all teachers in his employ who are members of the retirement system as the aggregate amount of the normal and deficiency contributions for the year shall bear to the total compensation or salaries paid by all employers, except those who operate local district pension systems, to all teachers who are members of the retirement system.
N.Y. Educ. Law § 521