You are right. Anfängerinnen is the plural of the word Anfängerin.

This could be either:

Capitalized I is there because of gendering. 

It is to emphasize that both Male and Female persons are being addressed.

But normally you would write it like: 

    Anfänger-/innen


Or which is more likely: 

Capitalized I is there to emphasize that they are only looking for female students. 

Hard to say without context.