Was poisoning Kim’s yogurt necessary?

Near the beginning when Bryan attaches a sticky note to the yogurt in the convenient store where Kim gets her yogurt, he mentions that he added some chemical of sorts in the yogurt to ensure Kim feels nauseas having to use the bathroom. I don’t understand this.

If he was able to write a message (which read “Drink me now”) on the yogurt, couldn’t he have simply wrote more of a message along the lines of where for her to meet him and when (something like “meet me at your school bathroom at 10 am”). Or was he worried someone else might possibly see it?

Why go through the risk of making her sick? Also, how did he know exactly the time she would need to go to the bathroom and which bathroom she went to? I feel like this wasn’t clear enough.