Caught Between Duty and Desire

I'm a high school teacher, dating someone casually, but we've never taken it to the next level. She's smart, funny, and every time I see her, my heart skips a beat. We've been texting late at night, sharing jokes and getting into some playful flirting. It's becoming hard to keep things professional during the day. Last week, she sent me a picture in nothing but her underwear. Just seeing it made me feel conflicted, there's this teacher-student dynamic that's always been taboo, and now I'm struggling with how far to go even though she's not one of my students anymore. She wants more from me, and the way she looks at me...it's like a switch flips inside me. Am I being unfair to her? Is this just a phase? What do you guys think?