Personally I would not actually use a converter on a 75-300mm lens. These tend to be acceptable quality on their own, but not really suitable for converters.
Teleconverters should really be kept for use on good quality prime lenses.
I have used a teleplus 7 element 2x converter and it seems to be as good as you can expect.
Tamron and Sigma also have a good reputation for independent manufacturers.
I now stick with Canon's own converters, but then I have more money now!
If you want my general advice on photography, I would say use the kit you have. Learn how to get the best from it you can, and don't concern yourself too much with acquiring extra equipment.
Too many gizmos and accessories can detract your mind from what you should be thinking about - the photograph.
Firstly learn about composition, the rule of thirds etc., books will help as reference.
Then learn about exposure, depth of field, perspective etc.
Look at lots of good photographs and analyse why they are good.
When you have a better understanding of how to take good photographs, then you will know better what equipment you need to take the photographs you want.
The class is a very good idea, it will be the best investment you can make at the moment.
Hope this helps.