Some tips for composing something for a level 3 orchestra.?

Hi! I am a soon to be 8th grader. My orchestra teacher recently found out that i enjoy composing. i have never written anything but pieces for piano and violin. I was eager to take on a challenge of writing something for my school orchestra. I have won several compostition contests with my compostitions. i was just wondering if you could give me some tips on how to i write for orchestra. I am a violinist and pianist. I am in susuki book 6 for violin and for piano, somewhere in the level range of bachs invention no. 8. Could you give me a few pointers for writing a cool piece for a string orchrstra please?