Peter Buxton had just turned thirteen. He was quite tall for his age and well-built. He had short blond hair and blue eyes. This morning he was wearing a clean white shirt tucked in and his navy school tie with yellow stripes was neatly tied. Instead of his usual navy school trousers, today he wore a new pair of smart grey flannels, fastened with a black belt. His black shoes were well-polished.
As he was walking to school a girl in his class came up behind him. She was slightly shorter than he was and had long dark hair. Her name was Rachel Pearson. She was wearing a white blouse and a navy skirt.
‘Good morning, Mr Buxton,’ she said. Peterblushed.
‘Good morning, Miss Pearson,’ he replied.
‘You look very smart today,’ she said.
‘Thank you,’ he replied. He hadn’t overcome his uneasiness, knowing that she was looking at him admiringly, as if trying to work out what was different about him.
‘Mr Buxton,’ she said, ‘have you got some new trousers on?’
‘Yes, Miss Pearson,’ Peter replied.
‘They’re lovely,’ Rachel said. ‘I think they suit you.’
‘Thank you,’ said Peter. ‘I got them for my birthday.’
‘Your birthday?’
‘At the weekend.’
‘I didn’t know.’
Peter was at a loss for what to say. He and Rachel were friends, but he didn’t know her all that well. The trousers had been part of a suit his parents had bought him for his birthday. He hadn’t had occasion to wear it yet, but decided to wear the trousers to school for a change, as boys were allowed to wear grey as well as navy, though very few did. He hadn’t really expected anyone to notice. No-one else seemed to for the rest of the day. He didn’t really think about Rachel again, until he was at home in the evening, when an envelope was posted through the front door. While he was watching television in the front room with his younger sister Louise, their mother came in and handed Peter the envelope.
‘There’s a card come for you,’ she said to him. It was addressed to ‘Mr Peter Buxton’.
My humble opinion:

* If you whittled down your opening paragraph a bit, readers would find it more interesting. Perhaps no one would need to know so many details about Peter's outfit.
* Complex-compound sentences help too.
*Trust your reader's judgement with a few details.

Peter Buxton had just turned thirteen but he was quite tall and well-built for his age.
This morning he wore...

BUT let's wait and see for a native speaker's suggestions.

Tom