That's true for adults who know what they're doing. Teaching the compression methods to kids would be extra dangerous (as shown in part 4 when archnobles were dropping like flies when they first learned compression, needing multiple teachers to help prevent any deaths) (part 4)
Yes because they suck at teaching it as I said most of these kids don't do anything that was discussed so they have no frame of reference for what is supposed to be done they don't see "reducing a potion" or "squeezing fruit juice" as things that reduce space. These are also poor techniques and the one that sounds like it would be good (pushing more into it which I think was Raufens) is actively dangerous because it's not reducing but forcing more into yourself again which is dangerous. It's about using he correct technique when teaching them. There's 3 ways to increase pressure 1st increase force (which the Raufens technique does) or 2nd reduce usable space (which is mynes 3rd technique of slamming the box shut) and 3rd reduce empty space and this one is the one that myne uses but none else did and is also the one that SHOULD be the safest as it doesn't add mana (that would be more difficult to control) but just controls the mana itself to increase density