When the fight kicked off, Malfoy attacked Harry and Harry attacked back with which spell/charm?
I'm uncertain because they didn't speak the words of the spell.
In the book it was Levicorpus, the same spell used to hoist Snape into the air in Snape's flashback
Malfoy wheeled round, drawing his wand. Instinctively, Harry pulled out his own. Malfoy’s hex missed Harry by inches, shattering the lamp on the wall beside him; Harry threw himself sideways, thought Levicorpus! and flicked his wand, but Malfoy blocked the jinx and raised his wand for another –
‘No! No! Stop it!’ squealed Moaning Myrtle, her voice echoing loudly around the tiled room. ‘Stop! STOP!’
There was a loud bang and the bin behind Harry exploded; Harry attempted a Leg-Locker Curse that backfired off the wall behind Malfoy’s ear and smashed the cistern beneath Moaning Myrtle, who screamed loudly; water poured everywhere and Harry slipped over as Malfoy, his face contorted, cried, ‘Cruci—’
‘SECTUMSEMPRA!’ bellowed Harry from the floor, waving his wand wildly.
Harry Potter and the Half Blood Prince: Chapter 24 - Sectumsempra
In the film, they appear to be using wordless stunning spells.