Southern Star Ranch has a great hunter/jumper trainer. It is south of Green Cove Springs, so it is a bit out of the way, but well worth it in my opinion. Not only is the person who gives lessons great, but the barn is just very friendly and easy going.
Southern Star Ranch - A Family Ranch with Wholesome Values
There is another barn on CR-210 that is very good as well, but the lessons are more expensive.
www.bella-donnafarminc.com
Good luck - my daughter rides and we had a heck of a time finding English lessons. She did lessons at Southern Star and camp at Bella-Donna and I felt both were great for lessons and very safe.