Can stress trigger an acne flare up?

Yes. Lots of things do this. Manage the acne. Stress is part of life. If topical benzoyl peroxide is not working, get with your physician who can add something.
Yes. I don't know ANY illness that is NOT aggravated by stress. We all have stress, but react to it in different ways- some have flares of neck or back pain, asthma, headaches, intestinal problems etc., based on our individual constitution. Try to reduce stress with exercise, meditation, adequate rest, healthy diet, talking with friends and family, a good multivitamin, magnesium, herbs like Tulsi tea.
Stress — Acne. Psychological stress can probably worsen acne. In several studies of students, acne severity appeared to worsen during times of increased stress. http://www.ncbi.nlm.nih.gov/pubmed?term=12873885.
Yes. Stress has been proven to be one of the reasons to get acne worse. Having a serious test, job interviews, stressful situations; will make acne worse as stress will trigger this worsening. Hope this helps.