Seniority after availing General Transfer

Seniority in respect of service experience will be counted from the date of your first joining, if you are in continuous service.
As one gets General Transfer only once in his/her service life, the question of considering in respect of seniority in case of General Transfer does not arise here, as @sourishjal mentioned above.
Still we need to know what is the context for which you are confused regarding your seniority.

It is all about maintaining the Acquittance Roll in the school. That means the downloaded Salary Bill and the Acquittance Roll should be in the same order. Generally the newly appointed teachers (in the present school) are put after the already existing ones. So your name will come in the Acquittance Roll as well as in OSMS in WBSED Portal according to the joining date in the present school. But that does not mean that you are junior than one who is ahead of you in AR of the school or OSMS.