My intented teach for most is "on plane with the pitch" which results in just slightly above "level". I do not teach to adjust that angle based on the pitch speed (off speed vs FB, for example) but find that the eye and brain automatically make that adjustment. I understand that the actual pitch plane will be slightly steeper than perceived and I think that works out OK.
In drills, we want hitters hitting hard line drives. Depending on the adjustments a given hitter needs to make, or their strengths/weaknesses, we may adjust the sights slightly up or down. We have a line drive contest that we utilize frequently in the cages. A player "scores" when the ball is hit hard and the Hi/low marks are bottom of the L-screen and top of the net beyond the L-screen (so probably about 25-30 degrees). Ideally, the bottom would be raised to middle of L-screen but we do this, in part, to make for easily delineated parameters. When I work individually with hitters, I will adjust specifically to the hitter's capabilities, natural swing and other factors.
One area you and I disagree on - I do think an HR should be a mistake as opposed to an objective for most HS hitters. As Golfman pointed out, attempt to lift and/or hitting fly balls almost always results in outs. A more successful/appropriate plane thought I will use is "dent the fences". "Drive it hard". "Where it's pitched" (which, BTW, has not been a factor in this conversation in regards to launch angle and it needs to be). "Drive the gaps". When you combine these swing thoughts with teaching to be on plane with the pitch (keeping the barrel in the hitting zone longer) and seeing on-field results, you end up with hitters who hit hard line drives and tend to miss ever so slightly under the ball (eventually becoming subconsciously purposeful backspin) with low line drives and gap shots as the objective. A squarely hit line drive, albeit not always over the IF, is usually a good result. A very slight miss under is usually a good result. A very slight miss hard one/two hopper is usually a good result.
You said.. But I could never be persuaded to the point of 'level swing' which i believe leads to failure at just about any level. There is a difference between "level swing" and "level" teach. As I mentioned in previous post, often, if your "teach" is level swing, the hitter's eye adjusts to pitch plane and result is a swing that is on or very nearly on pitch plane. Whereas, if you "teach" an upward swing or a swing on pitch plane, the result is most often something more upward than pitch plane and undesirable, IMO - the results I see are excessive rollovers, popups and otherwise inconsistent contact.
As others have pointed out, just "see ball, hit ball hard" has a lot of merit. This should be the objective once mechanics are in reasonable order. We also do drills that focus solely on getting the sweet spot to the ball. This is, I think, maybe even more important than launch angle at this level - becomes more of a given at levels beyond HS.
This year, I probably have two players who are strong enough to benefit from trying to drive with lift. I have another who is strong enough but currently overly exaggerates staying inside to the extent that he gives up too much of his potential power so that needs fixed before we go toward lift.
I do my best to keep up on all new information, data, mechanic teaches, game strategies, etc. But I also realize that most originates from the top (MLB). So, I am careful to examine what applies and to what extent to the HS game that I coach.