Python POST request says "Not Acceptable!"

My school has a website that students can sign up for classes during a study period and I forget all the time. I'm trying to make a script that logs in and signs up for me. This is what I have:

import requests

payload = {
    "username": "",
    "pwd": "",
    "User-Agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/42.0.2311.135 Safari/537.36 Edge/12.246"
}

with requests.Session() as s:
    p = s.post('http://www.bloomingtonsouth.org/plus/login.php', data=payload)
    print(p.text)

    r = s.get('http://www.bloomingtonsouth.org/plus/student.php')
    print(r.text)

The output: "Not Acceptable!

Not Acceptable!

An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.

Not Acceptable!

Not Acceptable!

An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.

"

What am I doing wrong?