This time watch the organization, the form. Think about the form before you mark it as inadequate. If it is inadequate in the final analysis, you will at least be able to say you have given it a fair chance, and you will have much better advice to offer for improvement. Spelling and grammar are important, they contribute to or detract from written communication. But they are not writing.
Science does play a role in art.
A painter must know about pigments and light. But a painting is far more than those mechanical considerations. When you are marking spelling and mechanics and grammar, bear three things in mind. First, remember the present abilities of the child. If a seven year old makes a stab at a hard but appropriate word, be more ready to praise his vocabulary than to correct his spelling.