I added more detail and I like it more now.

I'd investigate YOUR halls ;)

'Kirigiri!' a familiar voice called out behind she turned around, Ishimaru was walking towards her.

'Is something wrong?' Kirigiri asked, thinking over all her recentmost actions, trying to figure out if she'd broken any rules.

'Yes, and I have a request for you!'

'Me?'

'Would it be a problem for you to help me out with something?'

Well that sure came out of nowhere

'And what would that be?'

'Someone has made a horrible mess in the music salon. They looked for the rule-breaker but couldn't find anything so they're thinking of dropping the investigation! They're going to use our school's resources to fix the damage! That is unacceptable!'

Kirigiri was quiet for a moment before saying, 'You aren't turning to me because of the situation with the headmaster, are you?'

'Of course not! I am simply asking for help from a classmate.'

'Very well, I'll meet you in front of the music room in two hours.'

'Thank you!' Ishimaru said with a slight bow and they parted.

Kirigiri arrived at the appointed place exactly on time. Ishimaru was already there. He glanced at his watch when he saw her, approving of her punctuality.

He unlocked the music room and they got to investigating. Well, Kirigiri was the one investigating, Ishimaru was just observing her actions.

About an hour later Kirigiri was ready with the examination of the 'crime scene':

Only the guitars were out of their places; it looked like someone had tried to adjust the strings of two of them. Another guitar had a big scratch on the back. Not to mention that there was some strange dust and wrappers on the scene. The most concrete piece of evidence was the few bright red hairs on the floor that probably only Kirigiri would be able to see.

The culprit was obvious, though the motive was still unclear, so they set out to find them.

They found their prime suspect and Hagakure in the cafeteria. The latter went pale when he saw them, letting them know they were on the right track.

'Kuwata, where were you last night?' Kirigiri asked her suspect.

'Huh? In my room, of course.' Kuwata answered and Hagakure looked even more nervous.

'Is that so... Then why did we find a certain part of the music salon in horrible condition, with your hair on the scene?'

'What? You've got the wrong dude, I was in my room all night.'

'Then you'll give me some of your hair for comparison.'

'Like hell! I didn't do shit to those guitars-'

'Oh? I never mentioned guitars.'

A tense silence hung in the room while Kirigiri and Kuwata were glaring at each other.

'Alright, alright!' Hagakure suddenly interrupted the silence, making everyone turn to him. 'We were in the-'

'Shut up!' Kuwata tried to stop him.

'Let him continue.' Kirigiri cut in.

'I bet Leon on a hamburger that he wouldn't dare to get in the room without permission, 'right? Now I owe him one-'

'Both of you owe much more than fast food to the school for the damage on school property!' Ishimaru interrupted him.

After more futile protests, the justice duo escorted Kuwata and Hagakure to the headmaster.

'Say, Kirigiri...'

'Hm?'

'Do you think you could... teach me a thing or two, so that I can do this on my own and not bother you with such offences?'

'It would take more than a thing or two, but I guess I can see what I can do.'

'Thank you!'

How do you expect to outrun Ishimaru when he's already there