In the fifth movie we see that it is possible to practice a Patronus Charm without using it on a Dementor. When Lupin starts training the Patronus Charm with Harry, he immediately lets him try it on a Boggart, which causes Harry to pass out.
So why didn't he start similar to how Harry taught it to Dumbledore's Army, given how much safer that method was?
This is addressed in the source novel. Harry wants his army to practice on a Boggart, but he doesn't have ready access to one. He feels that producing a Patronus in a well-lit classroom isn't nearly the same as producing one under pressure of attack. This also explains why Lupin used a Boggart with Harry in the first place, to more accurately simulate an assault by a Dementor.
They had finally started work on Patronuses, which everybody had been very keen to practise, though, as Harry kept reminding them, producing a Patronus in the middle of a brightly lit classroom when they were not under threat was very different from producing it when confronted by something like a Dementor.
‘Oh, don’t be such a killjoy,’ said Cho brightly, watching her silvery swan-shaped Patronus soar around the Room of Requirement during their last lesson before Easter. ‘They’re so pretty!’
‘They’re not supposed to be pretty, they’re supposed to protect you,’ said Harry patiently. ‘What we really need is a Boggart or something; that’s how I learned, I had to conjure a Patronus while the Boggart was pretending to be a Dementor –’
Harry Potter and the Order of the Phoenix - Chapter 27: The Centaur and the Sneak
Harry specifically wanted to learn how to cope with the Dementors’ effect on him, since Dementors were assigned to guard Hogwarts after Sirius Black escaped from Azkaban.
“You made that Dementor on the train back off,’ said Harry suddenly.
‘There are – certain defences one can use,’ said Lupin. ‘But there was only one Dementor on the train. The more there are, the more difficult it becomes to resist.’
‘What defences?’ said Harry at once. ‘Can you teach me?’
‘I don’t pretend to be an expert at fighting Dementors, Harry – quite the contrary …’
‘But if the Dementors come to another Quidditch match, I need to be able to fight them –’
Lupin looked into Harry’s determined face, hesitated, then said, ‘Well … all right. I’ll try and help.”
- Harry Potter and the Prisoner of Azkaban, Chapter 10 (The Marauder’s Map)
Equally important as learning the Patronus Charm was being able to actually cast it without passing out in the presence of a Dementor.
“Lupin grasped the lid of the packing case and pulled. A Dementor rose slowly from the box, its hooded face turned towards Harry, one glistening, scabbed hand gripping its cloak. The lamps around the classroom flickered and went out. The Dementor stepped from the box and started to sweep silently towards Harry, drawing a deep, rattling breath. A wave of piercing cold broke over him –
‘Expecto patronum!’ Harry yelled. ‘Expecto patronum! Expecto –’
But the classroom and the Dementor were dissolving … Harry was falling again through thick white fog, and his mother’s voice was louder than ever, echoing inside his head – ‘Not Harry! Not Harry! Please – I’ll do anything –’
‘Stand aside – stand aside, girl –’
‘Harry!’
Harry jerked back to life. He was lying flat on his back on the floor. The classroom lamps were alight again. He didn’t have to ask what had happened.”
- Harry Potter and the Prisoner of Azkaban, Chapter 12 (The Patronus)
By the third try, he was actually able to stay conscious and produce a Patronus.
“He pulled off the lid of the case for the third time, and the Dementor rose out of it; the room fell cold and dark – ‘EXPECTO PATRONUM!’ Harry bellowed. ‘EXPECTO PATRONUM! EXPECTO PATRONUM!’
The screaming inside Harry’s head had started again – except this time, it sounded as though it was coming from a badly tuned radio. Softer and louder and softer again … and he could still see the Dementor … it had halted … and then a huge, silver shadow came bursting out of the end of Harry’s wand, to hover between him and the Dementor, and though Harry’s legs felt like water, he was still on his feet … though for how much longer, he wasn’t sure …”
- Harry Potter and the Prisoner of Azkaban, Chapter 12 (The Patronus)
