Firstly - don't let it psych you out. This is a very common issue (and I use the word issue very hesitantly).
A friend of mine was in the exact same position as you this time last year. In fact, she finished one SAC for the entire year! Timing really was her struggle. She often typed up essays, taking hours to perfect them. She just worked at it and slowly her time went down and down

. She never wrote a full piece in less than seventy minutes iirc, in fact, she didn't conclude any piece on the exam and she got a 50.
Your teacher's suggestion is a good one. Both my friend and I did this (however I rarely needed to, timing was one of my biggest assets, and served me well when I fucked up in the exam lol). I would also recommend handwriting perhaps every second essay. It's important to write it on a computer and feel like you're writing it perfectly, but handwriting is also useful. Don't give a time limit, but time yourself. Just try to finish as fast as you can. Legitimately. Don't be like "oh I don't have a time limit four hours is as fast as I can". Hard out, race yourself like your life depends on it. It will take some discipline.
Once you've done this race with yourself. See the time. Then take five minutes off. Your next essay, that's your goal. Then your next essay, maintain that five minute shave. Then shave another two and a half.
Conversely, think "I will write 500 words in an hour". Do it. Then, 550 words. Etc. However I prefer the first approach. I often sat practice SACs to time at home prior to a real SAC (or sometimes wrote practice SACs in class and aimed to do it in an hour. Was good in real SACs because it was like I had double the time).
A habit I also had was to write down the exact minutes it took me to write each paragraph. For a Twelve Angry Men essay, on average, it would take me about eleven minutes for the intro, around thirteen minutes on the paragraphs, and then the conclusion never mattered because it took me like five minutes. (Quite general times, they ofc vary depending on conditions) but it gave me a mental limit. If I got to the ten minute mark and I was half way through my intro, I knew I had to beast mode another paragraph. It was all a game of mental give and take.
Also, move your hand quicker. One thing I found that helped timing was rushing from the start (tactically, not like an idiot that will make many mistakes). If you write quicker in the last ten minutes than in the first ten minutes... well, write that fast for the first twenty minutes and then you have breathing room for the rest of your essay!! That was a habit of mine in SACs. We also go two hours, so I would write insanely quick at the start and then breathe easy

.
Sometimes, sacrifice a perfect sentence for your timing. People fuck themselves over psychologically. Don't take fifteen minutes going "fuck oh no cry cry my grammar in this sounds retarded and I sort of have a solution but it's sort of only half good". Just take the fucking solution, even if it is half good, and fix it later.
As you get better at writing, maybe your word count will decrease.
Just, don't stress too much about the exam. It's a long time away. Without fail, Year 12s do not consider SWOTVAC in their calculations of their abilities. If you do it right, you can improve... by... infinity...
Happy writing
