Severus turned on her, blocking her way, and did his best to loom over her with as threatening an expression as he could muster; this was not a problem he had tackled before. He clutched the bottle to his chest. "Headmaster," he corrected her - student or not - and took one menacing step forward. "No, you're not. Which means I won't need to explain myself to anyone terribly important should something happen to you. Which it will, I assure you, if you don't turn around this instant and let me be. You're making a fool of yourself. Go."
But he wasn't sure what he would do, really, if she refused. He could hardly act on his threats in public, and he would rather have swallowed the entire contents of the bottle he was holding than let her follow him home, even to whip up an antidote. This must have been what it felt like to be at a loss. How novel.