As learner of liszt said, mnemonics are the best way to go for remembering the orders of flats and sharps. I apologize, I'm not really creative tonight, but you might want to have them memorize a different odd statement for the flats as it's more challenging to ask them to remember the sharp order backwards for flats. Also remember that the last sharp is "ti", and the second to last flat is "do", except in the case of F major. As far as technique goes, you've got a much harder problem. I'm assuming if the kids have to do four octaves (sorry, I don't really know AMEB, I did NYSSMA) that they're going to have to perform them at a decent clip. For that I refer you to
https://www.pianofundamentals.com/book/en/1.III.5.1 for a reading on scale technique (there are links at the bottom of the page to move on to the next section), and
https://www.pianopractice.org/ for the TO/TU videos mentioned in the reading. Using the techniques described here, with a reasonable amount of practice your students should be able to achieve speeds of 120 and faster (I'm hoping for grade four you won't need more than 120, but hey). I've personally tried these methods, they work, you may even be teaching them already and I just don't know. Unfortunately, this is also time consuming, but it is worth it, I promise. If it won't be ready in time for the exam, well, my personal opinion is that the exam should be sacrificed in the interest of learning, but it's your call.
Other things to do: Have them sing scales on "la" or solfege so that they can hear in their minds what a scale will sound like on any given pitch. Practice hands separately! This is probably why they can't go down; their left hand simply isn't proficient in moving down the keyboard. It's much more complex than it is when the left hand comes up, and therefore needs more attention. Match the scales to the repertoire they're learning, and show them how the melodies/bass in the piece are made up of pieces of the scale and of arpeggios in that scale. Group the notes into chords ex. C,D,E and F,G,A,B in C major so that they can visually see these different fingering groupings and hand positions in the scale. Do NOT neglect the function of the arms in scale playing and even the torso in positioning the body as the scale moves up and down the four octaves. Play rhythmic, articulation, dynamic variations, have them compose melodies using parts of the scale, anything you can come up with. Make scale playing fun and productive. Students want two things: to have fun, and to see progress. You must do your best to make sure they are always going somewhere. Think about it: would you practice if you didn't think you were getting anything out of it?
BTW: If you think this is too complex, then break it down and be creative. However, please don't just accept the standard methods of endless repetition without attention to movement. If you're going to teach them scales, you might as well teach them the right way to play them the first time. I have a friend who's been in a sling for a week and a half because she developed forearm pains from trying to speed up the thumb under method for playing scales. Now she's freaking out and has a recital in two months. Just don't put your students there. You need thumb under, yes, but it needs to become the exception rather than the rule. The thumb can be played just like all the other fingers, using the muscles in the forearm, and it should be played that way. The muscles in the hand are slow and clumsy, and only for use when the tempo allows for complete security and relaxation.