Don't underestimate the value in a basic voice class. A strong foundation is paramount in anything you do. Also, you mentioned a lisp, I understand speech impediments cause I had some strong ones when I was younger. Though they are gone, I still feel their presents in my mouth and if I rush during speech, they can tumble out and soon my words are mush.
Unless you have something seriously structurally or neurologically wrong, you should be able to correct it. Don't give up, it is possible. If you are wanting to develop your voice in a serious way, you need as much control over your instrument as possible. Maybe you could find youtube tutorials to supplement your learning. Ask your teacher for guidance, usually they love to help.
Lastly remember that your own unique voice is your best commodity. Incorporate your lisp in an artistic way if possible. Sometime people ask me if I am bilingual or some interesting international person, I'm not, but my unique speech impediment past seems to add something. I wish you the best, keep singing. It is good for your soul.
martha