At least in respect to the tooltip for cantrips - cantrips are almost always all-or-nothing, unless you have a special perk that changes this. Just as a fighter doesn't get any free guaranteed damage when they use their unlimited resource (swing their weapon, doing no damage to anything if they miss), so casters don't get any free guaranteed damage when they cast a cantrip. If you want guaranteed damage, you use your limited resources - a spell of 1st level or higher.
The writing style for 5e (I know, It's almost pointless me referencing it at this stage...) is that spells do what they say they do: If a target takes half damage even on a successful save, the spell says so. If it doesn't say so, then they don't. Spells don't tell you what they *don't* do, they just tell you what they *do* do. So in the case of a cantrip like acid splash, it tells you: targets have to make a saving throw, and if they fail they take damage. That's it; that's all the spell does - if it did anything else (such as apply half damage on a successful save) then it would say so, as many other spells do - if it doesn't say so, it doesn't do it.
For the rest though, yes - when you choose your school, you *should* get a perk that means that, as an evocation wizard, you can ensure your allies automatically succeed their saving throws against your evocation spells, and that if a success would cause them to take half damage, then they instead take no damage. If that's not happening, then it's worth reporting.
