I recently asked out a classmate through text and she seemed interested enough, but she flaked at the last minute. After the time for our date had passed and she hadn't answered my texts, I realized she wasn't going to show up. Later on, she texts me apologizing that she couldn't go because of her boyfriend.

This factor showed up out of nowhere and caught me by surprise bc she seemed interested in the date idea and never gave any indication of being in a relationship. What gets me even more confused is that I never actually befriended her, so it's not like she would have thought it was a "friendly" date. Even more surprising is it's the second time this exact situation happened to me, so I guess the lesson is that I have to show more intent and be clearer and escalate more beforehand.

Question is: how should I reply to her? I know there's no point in being angry or butthurt, esp. since I shouldn't have expected too much beforehand. I don't mind texting back and I know it will seem childish if I just ignore her, but when I do respond, should I acknowledge that I thought she was single so I at least make things clear now? Or disregard the bf in my response, as if nothing happened?