It can be both. The question is necessarily asked in relative terms. In other words how high do u mean? This can only be answered in relation to something specific. Next thing is to take into account the person(s) involved in the expectation. How mature are they? How skilled or experienced are they. Of course it goes without saying that too high and too low are negative expectations. Those however are also in relative terms. Make sense?
It is a high goal set for something. It can be both good and bad. As long as the expectation is within a reasonable limit but is still high it is good. If it is set so high that it is unachievable that is setting someone up for failure and that is bad.
High expectation ... theres big difference between positive and negative... For example when you have a high expectation to something it is set that your expectation or goal is something perfect or things that you want successfully and if it happens you dis not achieve and did not meet your expectation the way it was design incomplete or unsuccessfully your goal that became negative because the goal was not successfully achieve... ;)