How do you politely say "NO!"?

I was 20 minutes late for my Korean class on Monday. As I was riding up the elevator to the 6th floor to my Korean class the 40ish year old Korean woman in the elevator started talking to me.
"Oh, you're studying Korean, right?"
"Yes." I replied.
"I will help you. I want to learn English, so you can help me and I will help you. Give me your phone number" and she shoved her phone at me so I could type in my phone number.
"uh.... ok... " I replied, not knowing what to do. Frankly, I don't need a conversation partner with someone this pushy. I put in my phone number. The doors opened for her to get off at the 3rd floor, but she followed me up to the 6th.
"Let's meet on Saturday, at 7:30. I'll help you study."
"Uh.... I have Korean class then. " which isn't completely the truth, but it's not completely a lie either...
"Ok, I'll call you later. My name is ㅇ아ㅣㄹ." (actually I don't remember her name/ don't care) and she jumped back into the elevator and I went to my class that much later because of this annoyance.

Why does this woman feel as though she can so abruptly assume that I need a conversation partner? I have plenty of people that I can practice with and I don't need some stranger shoving a phone in my face to practice with. I guess I shouldn't complain. She was trying to be nice (/ get free English lessons with someone who can speak some Korean) but this is just so rude! If she calls, maybe I'll tell her that I work crazy hours and can't meet or something.... or that I'm leaving the country, since I am in a month anyway. ^^