Rachel Pearson, aged thirteen, was walking to school. She had long dark hair and wore a white blouse and navy skirt. As she turned a corner she saw someone walking in front of her. At first she didn't recognise him, but then she recognised the bag he was carrying. It must be that boy in her class, Peter Bumby. He always carried his school bag like a brief case rather than over his shoulder. He wore his white shirt as usual, plus his navy school tie with yellow stripes, as she saw when he turned round after hearing her approach. Today however he wore a brown checked flat cap, such as she had seen old men wear, concealing his neatly brushed blond hair at the front. Also, instead of his navy school trousers, he had on a smart pair of grey flannels, fastened with a brown belt. These too made him look older. His black shoes were well polished.
Good morning Miss Pearson,' Peter said rather awkwardly.
. Rachel smiled. Good morning Mr. Bumby,' she replied. He blushed. 'You look very smart today,' she continued. 'I like your hat.'
. Thank you,' Peter said, blushing.
'Mr Bumby,' Rachel asked, 'do you usually wear grey trousers to school?'

You can write, spell, punctuate, and even get a story going.
I am going to critique this, however: for a short story, especially the opening scene, there's about twice as much description of clothing as it warrants. You need those important first few sentences for the entire scene and flavor. You also have Peter blushing twice a few seconds apart. Keep an eye on your verbs and adjectives and try for variety.
If you by any chance are 13, you are doing just fine though! If you decided to write it in the first person (of a 13 year old), all this attention to looks and clothing fits. It's what teens do.

Needs to flow a bit better. 5 short sentences open the paragraph. Keep working on it.