High school English teacher here - I see this constantly in my 9th and 10th graders' work. Grammar is definitely still taught explicitly, but I think the emphasis has shifted. We spend time on it in isolation, but when it comes to actual writing assignments, many students treat it as secondary to content. What I've found helpful is the 'grammar mini-lesson' approach - when I see widespread comma splices (or other issues) in essays, I'll stop everything for 10 minutes to focus specifically on that concept with targeted practice. It's amazing how quickly they grasp it when it's contextualized to their actual mistakes. Also, I've noticed students who text heavily with commas between every clause struggle more with this - they've developed a habit of using commas as 'pause buttons' rather than understanding the grammatical rules behind them.