1. Trying to be too broad. You need to be very specific in choosing your title. For example, 'Piaget's Theory' is likely to be far too wide. Focus instead on a particular part of his theory, or even on a particular experiment. You need to get into some depth with your topic and you simply won't be able to with only 2500 words.
2. Too much common sense. Be very careful of making any statement that seems obvious. At this level you are expected to justify and substantiate everything (ie give evidence, provide references, etc). You are also expected to be critical, so you are more likely to get credit for arguing against the position that 'everybody knows' is true than for just accepting it. Don't make general statements like 'research shows' - be specific.
3. Confusing personal opinion with evidence. It is OK to give your opinion, but make it clear that it is your opinion, and what it is based on. For example, don't say 'schools should not do XXXX' without justification, since this really is just your opinion. You could say 'If schools do XXXX, it could result in YYYY. I have experienced this myself ...'. Remember, your own experience is a source of evidence, but must be acknowledged as such.
4. Over-generalising. For example, if you chose to write about the ways teachers think about intelligence and the effects this has on learners, you might want to be critical of a lot of school practice. But be careful not to make statements that go beyond what you can justify. Don't say 'all/most schools do XXXX' without some evidence. However, you could refer to an event you have seen in a school or something that happened to you as an example to illustrate that some schools do XXXX.
5. Inaccurate referencing. You should know the rules about this and it is important to stick to them. Basically you should give a brief reference in the text (surname, date and, if it is a direct quotation, page number) for anything you refer to. At the end, a bibliography should give full details of everything you have cited. More detailed advice can be found in 'referencing'.