How time code work in cubase 7.5 or newer

Hi ! I work a lot by using score editor with pedagogial purpose. The students play together with cubase straight from the screen. I have been doing this long time but resently I started to use backtrack also… So then this makes the ”tempo detection” tool under the project menu very useful tool. After having detected the tempo in a back track and set the signature right it is just to play the song by ear into cubase with the back track and voala you have the score to mail the student ,or as I do, play it straght from the screen with the student.

My question here is if I do the above procedure then if I want to alter the tempo for the student the score goes berserk (the bar lines moves and it changes the lengths of the notes)

Lately I tryed ,now instead of using the very powerful tol ”tempo detection”, to instead start of with the score by loading with notes first and then after that imoprt the back track … and finally set the tempo track manually to syncronice to the back track.

Lots of work but possible… the strange thing is that after having done this procedure I can without problem change the speed of the song without all the score being moved (as described above) according to the barlines.

There must be either something in the midi that locks the bar lines or a setting i cubase that locks the bar lines to make it possible to change the tempo (ofcourse the back track then doesn´t syncronize with the slowed down score then but it is ofcourse not needed while it it just for practice(and not needed backtrack then…
some kind of timcode setting maby can be the answer.
wonder if anybody else have this problem or have an answer to this
Best regards Staffan (music teatcher)