My school was ranked 400+ something and 13 people got 90+ ATAR's.
Its mostly up to you and how well you rank, having a shit cohort can be beneficial in some cases, you can use it to your advantage to milk as many top ranks as possible.
150 is a good school rank?
How many people go there? I'm assuming quite a number get over 90, like 10 for a small cohort or up to 30 if its a larger cohort.
Subjects would help though, but I'd say top five is a good goal to cement it.