Young teenagers in middle school are at the age where the opposite sex stops becoming icky and suddenly seems interesting and cool. It's a time where there are a lot of questions, but also sometimes a reluctance to ask them. It's up to the parents to open up communication lines and make the conversation safe. When they have questions in the future, they will know they can ask you.
Middle schoolers will break up and get back together with the same person often. What may seem like a short relationship could be on and off for a year, which might make it difficult for parents to identify a more serious relationship. From his perspective, he is embarking on an "adult relationship." As a parent, make sure this new relationship is safe and with another teen his age. Dating someone older means added pressures for sex your teen isn't ready for.
This is a hormonal time where emotions fluctuate often and can be extremely fragile. Minor disagreements, real or perceived, between the teen and her boyfriend will seem like the end of the world. As parents, you have the benefit of experience and perspective. Be patient and understanding of your teen as she struggles for the same.
Experimenting with Sexuality
Children as young as 11 are kissing and touching nowadays, but at this age they are not emotionally ready for the physical aspects of dating. Your teen needs help drawing the boundaries of an acceptable level of physical contact. She is finding her way based on available information, such as the media. Parents are a better source of guidance what is and isn't ok for the couple.
School needs to be the priority, but your middle schooler may disagree. He is excited over his new girlfriend and wants to spend all his time with her. Homework and grades can suffer. This is a life balance issue he is just learning. Help him figure out how much time is needed for homework, then plan time with his date and friends accordingly. Remind your teen of the long term: education is important to the future, whereas the relationship might end.