Someone else recommended Majo no Jouken for your "complex" and I can't recommend it strongly enough! It's definitely not a rom-com but its an adventurous and sweet forbidden romance between a high schooler and his teacher with some pretty epic scenes, definitely among my favorites. (And it stars a young Tackey!)
Anego, Kimi wa Petto, Ohitori-sama and Sapuri all involve older women and younger men, but not high schoolers. All three are much lighter watches.
Absolute Boyfriend/Zettai Kareshi doesn't involve a younger man, but it does involve an office lady and her mail order robot. There's a similar "woman with an inexperienced man" element as a result...It's definitely a rom-com.
If you wanted to delve into Korean drama, Flower Boy Ramyun Shop is another story where a female teacher falls for one of her students. I Do, I Do is another kdrama, where an older, successful business woman has a one night stand with a much younger man and ends up pregnant. (Actually, anything starring Kim Sun Ah, the actress from "I Do, I Do" probably fits the bill for "older woman, younger man" romance. It's practically her trademark.)
If you're still looking for more, Koishite Akuma fits the bill (love story between a teacher and her vampire student) but I absolutely cannot recommend this show. It's short and I watched it all...but it's a pretty campy, poorly executed show.
