Is it possible to write publication with a BSc student co-author?
But according to our university licensing, consulting does not lead to any  authorship for the mentor.


You may want to consider double-checking what exactly that means. It's highly likely that your university does not forbid you to be co-author of a paper written under your supervision if you contributed intellectually to the paper, which is the case if the student uses your ideas. The rule most likely states or is meant to state that supervision alone is not sufficient for authorship.


  But I am afraid that journals/papers don't support BSc student authors that much.


How would they know that the student is a BSc student? The only case I've ever seen where this is mentioned is in IEEE journals, where some authors may have ", Student Member" attached to their name if they pay the student rate of the IEEE membership - but even then, it could still be a PhD student.

If you want the paper to be published at a good venue, you will most likely take an active role in the writing process, and in the resulting paper, it should not be visible what status the individual authors had.