What he's doing right now is its own form of training; he's sure that Iruka would agree, since he's currently giving a lecture on the subject inside the classroom.
"A shinobi must, first and foremost, be aware of their own body. Once you have mastered the inside, you're ready to reach beyond that. A shinobi can sense the chakra of enemies, and more than that, the movement of the world around them."
Kakashi does not move where he lies in the grass outside the classroom window, but he can sense the students inside. Some of them are paying attention. Some of them are twitching. One of them is nodding off until a very precisely aimed piece of chalk pelts him in the chest. "Iruka-sensei!" the little brat whines, and there's some laughter and the squeaking of seats as the kids turn to look.
"A shinobi should always be alert," Iruka says sternly, but not unkindly. He's quiet for a moment, probably gathering his thoughts. "You should know your environment, and be as aware of it as you are your own body; in time, you'll be able to sense the natural rhythm of things. You'll be able to feel a storm front, and you'll know when the birds will migrate for the winter."
Kakashi can picture one of the kids raise their hand just as easily as he can sense it. "Iruka-sensei, when are the birds going to fly south?"
Iruka is quiet again, and Kakashi can almost taste the currents of chakra that say Iruka is expanding his senses to listen to the world around him. "Not yet," Iruka says, almost meditative. "Another week."
Kakashi pegs it at five days, personally, but it's close enough for the rugrats, who won't master this kind of fine-tuning of senses for years.
Iruka continues with his lecture, and Kakashi focuses on him. He can hear the sound of the floorboards as Iruka shifts his weight before turning to write on the chalkboard. Iruka's ponytail creates eddies of dust motes in the air, and the vibrations of his voice carry clearly across the room. There's a faint slump to his energy, as to be expected for this time in the afternoon, and Kakashi idly wonders if he's in the habit of drinking coffee to combat it, or if he does light calisthenics instead.
"That's all for today," Iruka says, and the room explodes into noise as the kids get their things together and stampede out of the room. But underneath all the hubbub, he can sense Iruka moving stealthily toward the window. But just because he knows Iruka is coming doesn't make him any less curious.
Iruka lifts up the window sash, and releases a mighty cloud of chalk dust over Kakashi as he bangs some erasers together. He looks down at Kakashi, and says pointedly, "I just told a classroom full of children that shinobi can sense their environment. Did you really think I didn't know you were here?"
Kakashi chooses not to cough, although he really can't do anything about his eye watering a little from the dust. He blinks a few times to clear it, and then says, "Oh, I was sure you knew I was here."
Iruka looks flummoxed. "Then why do it?"
Kakashi smiles at him. "Maybe I wanted you to pay attention to me," he suggests, voice laden with innuendo.
Iruka rolls his eyes. "What is wrong with you?" he says. Kakashi hopes that's a rhetorical question. "Anyway, don't do it again. It's distracting."
"Hmm," Kakashi says. "Well, I suppose. On one condition -- you have dinner with me the day the wheat-sowers fly south."
Iruka looks startled. "In seven days?"
"In five," Kakashi corrects. "You know, Iruka-sensei, maybe you need more training. Are you sure you don't want to join me out here? You can learn a lot when you lie down on the road of life."
Iruka sputters. "Oh, go to hell," he says, and slams the window shut.
"Is that a 'no'?" Kakashi calls.
Advanced training goes pretty far, but only someone who pays as much attention to Iruka as Kakashi does would know that Iruka is blushing without even looking at him.