Hey. I just have to say, I empathise with you completely. I started learning Japanese in Year 9 last year, and I'm not continuing to learn it at school (because I have other subject interests), but I still learn it at home. I remember being put into the class and people could read hiragana and katakana really fluently, along with some kanji, I know how you feel.
Assuming that you can read the characters well enough, if you don't understand a sentence, find words that you do understand and try to figure out contextually what it could mean. You can also practice this by reading conversations or subtitles for Japanese media, as usually you'll be able to deduce what is happening.
Since it sounds like your biggest struggle is the vocabulary, I'd suggest getting an app called DuoLingo. It's free on all mobile devices, and I personally find it works really well to build your vocabulary, since it mostly focusses on it. I use it to study at home to this day, and it has proved extremely useful. Other apps I've used are Duolingo and Drops, but I've always ended up liking DuoLingo the most. DL teaches you the most basic vocab to some of the most convoluted sentences.
頑張ろう、suskianna-kun!