Investigators say the alleged assaults took place in Nelson's van earlier this year. Nelson is a physical education teacher at Indian Trail Academy.
Nelson faces four counts of sexual assault of a student by a school staff member.
Bond for the father of five has been set at $75,000 cash.
A criminal complaint says the Kenosha Unified School District learned about the possible relationship through an anonymous call and letter.
The Kenosha News reports Nelson admitted sharing text messages with the girl, but denied the two had any physical contact.
The Associated Press contributed to this report.