I think this is a very good composition. However, I think you should be a tad more careful with the grammar. Also, one should not insert a space before most punctuation marks (eg. comma and full stop) in English.
Once upon a time , A children called Tom had a dream . --> a CHILD called Tom
It is a space explorer. --> A dream is a space explorer? Or is Peter?
His uncle , Peter , was a space explorer . He gave him a chance to study in space school .--> Is 'space school' the name of the school, or a type of school? If it is the former, then the first letter of each word should be capitalised. If it is the latter, 'a' space school may be more appropriate.
During six months , he had not made a lot of friends and got much knowledge of space . --> People don't usually say 'during six months'. Also, you should tell the reader six months of WHAT? During his six months in the school... might be better. Since you used 'during', not 'after', the perfect aspect is absent, so changing 'had not made' to 'did not make' might be better. Also, 'got' should not be used before 'knowledge'; 'gain' and 'acquire' would be better. As non-native speakers of English, we must check the dictionary often to ensure that we do not misuse words like 'got'.
Finally, I think 'much' knowledge can be changed to something more exciting, such as 'vast', 'deep', 'broad', etc (though you would have to add an 'a' before those adjectives if you choose to use them.)
One day , there had a group of ET coming . --> it should be 'there WAS'. Also, ET is actually countable so you can write 'a group of ETs'. In my opinion, a 'group' of ET sounds a little too weak for the purpose, so you could perhaps change that to 'legion' or 'platoon' to make it sound more powerful. A 'group' of aliens may not be capable of invading the Earth!
They attacked the Earth and human , so Tom had a decide to save the Earth and human . Since they attacked more than one human, we should pluralise it. You can also say 'humanity' or 'mankind' if it is the human race in general. 'Had a decide' is inappropriate; you can simply leave out the 'had a'.
One man thought that Tom was a rightful boy. --> Usually, we use 'rightful' when we are talking about something that is legally or morally legitimate, etc. It is seldom used to describe someone. Maybe you can say he is brave or kind or somthing?
He gave a spaceship and a protective to Tom .Tom had started his adventure .
First , he found a friend called Gary who was not attacted by ETs and planed how to pick out they friends or protect the Earth . --> I think you should change it to read, for example, 'a friend, Gary, who was not attacked by ETs' (or something along those lines). It is because right not, some people might interpret the defining relative clause as referring to 'Gary' rather than 'a friend'. 'Planed' is the past tense of 'plane', not 'plan'; it is spelt 'planned'. What did you mean by 'pick out they friends'?
They went to the station of ETs by spaceship. They met the master of those ETs and they had a promise .If Tom and Gary got a jewel and gave to them, ETs would go out . --> What is 'the station of ETs'? I think you should make it clearer to people who don't read sci-fi. I think 'had a promise' can be changed to 'came to a compromise' to make the meaning clearer. I think 'got a jewel' can be changed to something like 'found' because the jewel is hard to find; you dont' just 'get' it in a store or something. You should add 'it' after 'gave'. It should be 'THE ETs'. since they were mentioned before. Also, 'go out' is not clear; maybe you can change it to 'leave'?
Then , they went to find a jewel .But where was the jewel ?they ask Tom’s uncle , Peter, and Peter said “the jewel you wanted in the house beind the giant mountain” --> The first sentence could be changed to 'they went looking for the jewel.' 'But' should not be used at the beginning of a sentence; you could change it to 'yet' instead. Since you already mentioned Tom's uncle, you can simply write, 'They asked Peter, who said...' 'the jewel you wanted in the house beind the giant mountain' is a little strange. I'm not sure what you mean, but my guess is that you wanted to say, 'the jewel you wanted is in the house behind the giant mountain'.
They shocked , because the mountain was very high and nobody can came over the giant mountain. But they must find the jewel which ETs need . --> They WERE shocked. You don't need a comma before 'because' since it's an adjunct, not a disjunct. 'Come over' is not used like this; perhaps you can use 'cross' instead. The perfect aspect is present because Tom is planning to cross the mountain; thus 'nobody had ever crossed...' might be better. Again, 'but' is not used at the beginning of a sentence. You should write 'THE ETs needed' because the ETs were mentioned before.
I think your writing is very good, but it can be made better by tweaking a couple of mistakes here and there.
Anyway, these are just my personal opinions. I think you should get a 4 for this writing.
