There are not only psychological risks involved with dating an older man and trying to fit into an adult world precociously, there are also dangers of being exposed to drugs, alcohol, abuse, and an increase rate of sexually transmitted diseases and pregnancy.
If he was ignoring her, she was depressed, missing class, consumed with anxiety.
During the teenage years, young people are trying to come into their own and often rebel against their parents and other adults, which is why they often chose friends and relationships that their parents disapprove of, including dating older men.
I hate when I am working with a teenage girl and she tells me she is dating an older man, usually because I know that this relationship, while to her may be idyllic and dreamy, is more often a disaster waiting to happen on so many levels.
A lucky few end up in marriage, which is often proceeded by a child and continues with multiple children, poverty, physical and substance abuse. Let him know that if he really cares about her, he will abide by your wishes since you only have her best interest in mind and he should too if he cares about her.
Depending on the age of your daughter and the age of the man, it may be appropriate to get law enforcement involved.