Mr Clare has completely covered what I wanted to say (as usual).
It's clear you're extremely enthusiastic and intelligent, so don't take a request from your teacher to shall we say "dumb down" your essay / project as an insult.
By being able to summarise I think that you are showing an ability to point out the key points and cover them in an easy to understand way, rather than going in depth in to every possible thing and (quoting my history teacher) "waffling on". I think at the level your teacher wants you to write at , writing about the most important factors and going into depth is better than writing every single thing you know about every single possible factor, even if you go into depth.
Considering you are 13 and under, I somehow doubt your teacher expects or wants to read a 10,000 word essay. Stick to your parameters, being able to give the facts to the reader / marker in a clear and consise way is more important than endlessly expanding and elaborating.
If you're presenting this to a class of your peers, you need to keep them entertained ! If you have a class of people who like gory stuff, talk to them about what they like listening to, that way you will keep their attention.
For developing a point, I have always been taught to write a paragraph following a simple structure :
Point - What you are trying to explain, for example if the essay Q was "Why was there a revolution in Russia in 1905 ?" one of the points could be "Nicholas II was a weak ruler"
Content - Give more detail to the reader as to the reasoning of your point. Always imagine that your reader knows absolutely nothing about the essay subject. "Nicholas II wasn't ready to come to the throne in 1894, because of X, Y, and Z which caused him to be a weak ruler"
Evaluation / Content - How does this
relate to the essay question ? If it doesn't relate to the question, it shouldn't be in your essay. "Due to Nicholas II's weak but brutal autocratic leadership, the Russian revolution of 1905 was able to happen and this is why *bla bla bla*" - briefly summarise the points made in the content, never add anything to the conclusion / evaluation that hasn't been previously stated in the content.
I've waffled on enough

Good luck

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