I studied under Numael Pulido. That is, Numael has had a profound positive influence on my work. Like many other artists, I have "studied" under other teachers, but none have made much of a difference.
Shortly after I moved to Peterborough, New Hampshire, I fell in love with Numael's work that I saw in a local show. I called him (thank heavens he lives nearby) and asked if he taught. Fortunately he did and I took his classes for two years. Numael Pulido is not famous. And not all of his students started out to be serious painters.
Numael will not (under any circumstances!) do a workshop. And he requires a student to attend class once a week for at least a year. He also severely limits the number of students that he will take on. I feel extremely fortunate to have had the opportunity to work with him. Now Numael and his extremely talented wife, Shirley are dear friends.
When an artist's resume says that they "studied under" a teacher I presume that they mean that they took a workshop or lessons from that person. I also assume that they sincerely credit these teachers with an important influence on their work. Just listing a big name in the hopes that a little glory will rub off is kind of silly but I suppose that happens too.
I suggest that when anyone is shopping around for a good teacher, that they look at the teacher's work. There are a lot of "famous" names out there, but if you aren't crazy about their work....don't bother to sign up for a class. Also when shopping for a teacher look at the work of the other students...before and after if possible. If you don't see any improvement in their work, I would have second thoughts about the teacher's ability to teach.
Any artist can only teach you what they know and sometimes that isn't enough. A good teacher ought to be able to do something better than you can.
Also, sadly, some teachers are not willing to share their knowledge at any deep level. Even sadder is that so many students don't realize that they are entitled to leave a class with improved skills - instead of the all-to-common feeling that they are lacking the necessary talent to do good artwork.
Before you reject "adult classes" check out the teacher....someday you might be proud to say that you studied with them.