I'll try shed some light on this too..
Having high expectations can be a good thing. If you set yourself with a high goal, it would be natural for you to fight for it. This would entail working hard, making sure you can do the best you can and not making excuses for as to why you didn't/can't/couldn't do something. It almost serves as a motivation factor at times to strive for the excellence, and to make sure don't become complacent which could actually yield a unsatisfactory mark to you. If you didn't aim high, you wouldn't know how capable you are and could be left wondering how much more you could've done/achieved. They say, aim for the stars as if you fall short, it's better than aiming for the sky and just getting there.
However, there are some negative aspects to having high expectations. Firstly, if you don't initially start off on track to achieve them, it may leave you feeling deflated. This feeling of not being good enough, or feeling of failure so early may be detrimental to the long term goal and your future performance. Equally so, having these high expectations can actually also make you put unnecessary pressure (the negative type) on yourself to do well, and then possibly beat yourself up if you don't, which can once again spiral downwards.
Personally, sometimes try not to think about what you NEED to do or what other would expect from you. Rather, just put in the most amount of time and effort you can, and back yourself. Trust in what you've done, and take things as they come. It can help with the stress and the rest (grades and rankings) will simply take care of themselves provided you did what was necessary for you.