It's not bad, it just feels a bit heavy compared to a lighter styles. You add a lot of things that are implied, or over-explain things:
The sun burned hot above the coastal city of Ancio in the late summer. (I would lose 'in the late summer, and ask myself, do I need this at all before mentioning Mat. Can it be added in a shorter form later when he climbs on the roof and actually deals with the hot sun and the veiw of the coastal line?)
It was the day for a young sixteen-year-old boy called Mathiaz Rider. (why do we have this sentence at all? It steals the suspense out of the rest of the descriptions because we already got a boring spoiler)
He woke up this Sunday morning, knowing he would forever abandon the city he was raised in. (why do we need that? Cam we feed that while he is on the roof?)
Despite the heat, Mathiaz donned a leather jacket, a sports shirt, and beige jogger pants. He then climbed onto the roof of his orphanage with his favorite but unkempt sneakers. That was where he found himself now, enacting on his revenge and facing the unfair cruelty bestowed upon him. (now, this one is more like the beginning except for too much irrelevant details. I would just leave some, and use some stuff from the previous paras, starting your story with:)
Despite the late summer heat, Mathiaz climbed onto the roof of the orphanage, bundled up like it was the middle of the winter. He was no fool to leave a single item of value behind. The losers would pilfer it, because he was leaving today, and they were staying. Arseholes.
Fresh breeze chilled his skin under the mop of sweaty curls. The leather jacket plastered itself to the skin of his back, itching like crazy. Morning sun beat into his eyes, but he jutted out his lower lip.
He would not be denied his revenge.
Fingers shaking with the exertion of catching the cracks and groves in the old stones, Mathiaz dragged the can of spray paint from the waist-band of the jogger pants two-sizes too large. The entire world was going to know what he thought about this stupid orphanage, this stupid city and his whole stupid, unfair life.
The jet of blood-red paint erupted from the can's nozzle, overwhelming the ever-present smell of sea spray and rotting sea-weeds in the air. Waving his arm wildly in the air, Mathiaz enblazened the wall with the gigantic cry of his soul:
(now tell me what he wrote on the facade and let him reflect and feel through it before the cry of fire)