[Plugin: Post from site] can't write post if I have limited capabilities

I’m using plugin “Post from site”.
Every post written from the front end is not a standard post, but a special post type (I called it “project”).
I want to allow just a special type of users (a new type I called “Students”) to use it. students can’t write standard posts, but just “project” type of posts.

The problem is: when a Student goes to the front-end editor reads “You must be logged in to post.” and doesn’t see the editor. I think that’s because in the plugin code there is this line

if (current_user_can('publish_posts') || $options['allow_anon']){

so I changed the line (every time it appears in the code) into this

if (current_user_can('publish_project') || $options['allow_anon']){

but it simply doesn’t work (and I’m 100% sure the student has “publish_project” capability). Is there some other change I must do in order to allow Students to write from front end?
Thanks everyone!