Colin Creevey: Howlers in the Owlery
It's time to burn the weird plant outside the Owlery. Colin is the third person required to do it. Do not hesitate to cast Incendio and then find your way inside the Owlery.
When you're inside, Colin will begin to pass packages to you. You have to put them in right places using Wingardium Leviosa. After finishing that part, Colin will tell you Filch is coming and you must find other way to get out the tower. Climb up the stairs and pass through the door. In the room repair broken shelf (Reparo), climb onto it and jump through the window. You shouldn't have any trouble getting down to the ground, it just takes time (Harry is quite slow).
Dean Thomas: Sabotage the Clock Tower
The rebellion is getting serious. Notice three cogs hanging on right side of the clock. You need to take them off (yes, Wingardium Leviosa) and hang on pegs on left side of the clock, but in the right order, so their sizes match (take a closer look at the screen below). There are three more cogs on the top floor. Go there and pass them to Dean using Wingardium Leviosa.
Get back to Dean and put three new cogs in right places (look at image below).