It would change in many ways. They would go from being carefree high school sweethearts to having a lot of responsibilities. There would be more arguments as the stress of the situation builds up. But at the same time, they would also have deeper moments of love and understanding as they face this huge life change together. Jace might become more overprotective of Clary, and Clary would need Jace's support both emotionally and physically.
One typical challenge is secrecy. In some fanfictions, their families or certain school groups might not approve of their relationship. So they have to keep it a secret, which is hard when they're in high school and constantly around others.
In fanfiction about Clary and Jace's high school love, their relationship is often complex. It could start with a simple dare. Maybe Jace's friends dare him to ask Clary out, not knowing that he already has a secret crush on her. Clary accepts the date, thinking it's just a joke at first. But during the date, they find out they have so much in common, like their love for a certain type of music that plays in the local coffee shop near the school. This shared interest becomes the foundation of their growing love, as they start to spend more time together, sharing secrets and dreams.
One possible plot could be that Clary and Jace face a lot of social stigma. Their friends might be shocked and some might even turn their backs on them. They would have to deal with their families' reactions, which could range from disappointment to support. They'd also have to figure out how to continue their education while preparing for a baby.
It might progress slowly at first. They could start as friends, sharing lunches and talking about their classes. Jace might gradually open up to Clary about his dreams and fears. For instance, he could tell her how he feels pressured to be the best in everything. As time passes, they become closer, and their friendship starts to turn into something more, like when they hold hands for the first time during a school event.
One common theme could be the challenges they face in high school while dealing with the pregnancy. Another might be the reactions of their friends and family. For example, their friends could be either very supportive or judgmental.
Alec might also play an important role as a supporting character. He could be the voice of reason for Jace, making sure he doesn't make any hasty decisions. And Luke would surely be involved. As someone who has always been like a father figure to Clary, he would offer his wisdom and support, and maybe even help them figure out the financial aspects of having a baby so young.
Their first interaction could be intense. Maybe Jace, with his confident and somewhat brooding charm, catches Clary's eye in the hallway. She might be intrigued by his cool exterior. He could give her a lingering look, and she might blush. Then, he might say something cryptic like 'You seem different from the others here.'