Well this is a bit late and you should have finished your o levels by then but here’s some info from a frequent green tea drinker :

1. It only lasts an hour or Two , depending on caffeine resistance and what brand
2. It gradually starts and wears off
3. Powder ones not that effective, depending how brewed . I’d stick with teabags. 4.taste is a bit grassy , depending on type and strength 5.Since by the time this is posted your O levels are already over hope you did well . If you want to try out actual tea go to don don donki green tea section , get the loose leaf tea there , and also go to daiso and get their loose teabags and then size them porportionally . Brew them according to the instructions.

Sorry to not have seen your post in time , but hope this gives you some insight into studying for ur future exams and gives you something to do over the holidays:)