High school teachers are wrong.
One thing I dislike the way mainstream language learning in school operates is the focus on grammar. The dislike comes from my principle that language learning should be natural rather than artificial, that is, focus on getting your idea across rather than speaking with perfect grammar. I spent 6 years learning French in high school, focusing on practicing grammar and speaking without mistakes, and yet my French is worse than my Spanish, which I spent only 4 months learning.
When just starting a new language, learn the simple grammar and sentences structure, and focus on getting your points across quickly and fluently. Speaking fluidly without pause and getting your ideas across makes you sound MUCH MORE like a native speaker than someone who takes many seconds trying to think of correct conjugation.
A lot of times when I’m chatting with a Turkish native, I may occasionally make a few grammatical mistakes, but they will understand me. (Onlar beni anlıyor)
In other words, learning a language is a mean to an end. You learn so you can communicate with people, not because so you can speak the language perfectly. Therefore I ignore all but the most basic grammar when I learn a language.
This is, after all, how babies learn languages: prioritize on making themselves understood first, then proceed to fix nuances later when you can already speak confidently
(ps. I must say Turkish grammar is extremely logical. There are only few rules you have to follow, and are consistent, unlike Spanish/French/English grammar!)