Legal advice re teaching disciplinary.?

My partner left the school premises and did not inform the line manager, she did inform the receptionist and was out at a work related issue. She returned having missed no lessons however 2 days later her line manager asked her in future to inform him. Later that day the HT called her to the office to issue a disciplinary letter stating that the leaving of premises and information as to why was false. She has been told this is gross misconduct and can lead to dismissal. She has to continue working under this and is worried about going back to work. She has never had a warning but has had issues re the line manager previously as the union have been called in many times re the treatment given. The union have been informed and feel that while this is misconduct it is harsh to class as gross misconduct, the letter states she has brought the teaching profession into disrepute. Please Help!! This is her career and if dismissed she feels that she will never get another teaching job.