How do I start casual texting with a girl I’m getting closer to? (16M/16F)

Hi, I (16M) have been trying to get closer to a girl (16F) from my class over the past few months. At the beginning we didn’t really talk at all, but I’ve been trying to build at least a friendship with her. In the last few weeks we’ve gotten closer and started talking about more personal things beyond school. The issue is that recently we’ve started texting a bit on WhatsApp, but it’s always been with school-related excuses or reasons. Now that the school year has ended, we can’t really start conversations with a simple “hey” or casual chat, because we’re not at that level of comfort yet on WhatsApp. Even though in person we can talk more naturally, we don’t really do that over text without a reason. That’s why I wanted to ask: how can I break this barrier in these last days so we can start texting casually without needing an excuse? I’d like to be able to maintain normal conversations over WhatsApp during the summer, because otherwise we probably won’t talk much for around 3 months.

Also, if you have any advice on how I could gradually move this relationship in a more romantic direction, I’d really appreciate it.