This was not what he expected.

OS

Marcus believed that girls lived in sorta sisterhood of friendship and caring.

He believed that they would gladly help him and teach him, how to be a woman.

Marcus was barely a girl, a few short weeks. He doesn't keep it a secret but doesn't talk about it either.

There were possibly hundreds of things didn't know about being a girl. 

That girls could be just as bad as boys were, maybe worst.
As he err, she went to his/her first PE as a girl. As he/she started to undress and but on his/her first time, her gym clothes.

Two girls that knew of Marcus transfiguration, came up to him/her. 

They acted like Marcus was in disguised. 

But he/she was fully physically, biologically, genetically a girl.
They knew that he/she was completely one.

But pretended that he/she wasn't one.

They pulled open his/her blouse open to expose his/her bra and there were holding two teenage breast in the them.

Then they pulled his/her skirt up, and to expose his/her panties. And grabbed his/her crotch.

Damn, either its real down there, or good at hidden it. 

After a few minutes, she/he was half-naked. 

And the couch came in you got five minutes to get ready or else.

And you two do you have gym now.