The way to make someone else look good is first and foremost, to do your own job well - get work done on time, and accurately. Maintain confidences.
As to the boss, don't let him/her get blindsided. If there's something going on that they need to be made aware of, let them know! Can you imagine meeting with the school board and being asked about a particular transportation issue, and being totally unaware that there is even a problem? Conversely, don't bother the boss with every little thing. Run "interference," taking care of the little stuff yourself or finding someone else to take care of it.
Try to stay one step ahead. . . for example, if you think the transportation problem might come up at the school board meeting, don't just make sure the boss is aware of it - give them a report, data, statistics, what's being done about it - so the board understands that the problem is already being taken care of!
That's my take - I wasn't in education, but, looking out for the boss is something that's done in all professions.
that is a really interesting question. But I am wondering why you think that this is the purpose of many jobs.
From my point of view the main purpose of a job is to provide value to the company or organisation you work for.
And I would doubt, that making a principal look good is really adding a lot of value in general. It may be just the opposite, you may strengthen a weak boss with that.
So my advice is that you don't think so much about how you can make your boss look good but focus on how you can add value to the person you support and the company you work for.