Investigate (5)

"Since earlier, I've been hearing nothing but excuses coming out of your mouths." The principal said sharply. 

The two people hastily bowed their heads, not saying anything. 

"So you leave her alone in the infirmary Ms. Mae when the gymnast teacher asked for your help?" 

"It was my carelessness sir. But I made sure to lock the door and the windows when I left. She should not have any key to open the door from the inside at the same time, no one should be able to help her out of the infirmary without notifying me. It wasn't unusual for me to lock the door in the infirmary when someone is inside. What's more, I slip some sleeping pills on the water I let her drink and I made sure she was fast asleep."

The principal seemed to have a headache as he held his temples with one hand.