For english, it wouldn't be acceptable, since you have to write in a report format with sentences. However, for Science, Geography and History, it would be acceptable because it is a means of getting your information clear and concisely (since you don't have much room to write on the lines).
I frequently do that in Science, Geography and History for the 5 mark questions because I know there is limited space and if I use this technique, I could possibly cross out an irrelevant point and perhaps put it with a more relevant point...
In the 15 mark questions, I did it for my geography trials, but not my history trials, since the history markers wanted me to demonstrate that you can write in sentences and paragraphs.