Luna Lovegood: Swamping the courtyards
After speaking to Luna, cast Wingardium Leviosa on one of the benches at the courtyard and move it towards the right column (X marks the spot!). Climb up onto the roof and turn left. Repair broken part of the roof and move forward. There's another X on nearby tower. Cast Reducto on it and then cast Wingardium Leviosa on portable swamp hidden in the tower. Move swamp to the courtyard and climb down the roof. Put the swamp on the X mark next to the fountain. Now cast Incendio or Reducto on it and something will happen. :)
You'll have to do the same (well, not exactly the same) with other two courtyards. Let's start with Paved Courtyard. As the cut-scene shows, portable swamp is hidden on the roof. Climb up the stairs to the upper level of courtyard and fix the gutter in corner. Use it to climb higher. You'll have to use shelves and gutters to climb to the right place on the roof. Basically, try to move to the left, unless you can get to the roof itself. When you're there, cast Wingardium Leviosa on the swamp and throw it down to the courtyard. Then, as previously, cast Incendio or Reducto on it.
Transfiguration Courtyard is the last one. You'll meet Ginny there. She'll introduce you. The box is on the tree. When you cast Wingardium Leviosa on it, a few Slytherins will show up, including Crabbe & Goyle. You have to defeat them. It isn't quite hard, besides you have numerical superiority. Standard Expelliarmus & Protego combo should work. If you lose, you'll wake up in hospital wing and will have to fight again. In the end move the box to the right place and cast...You know what. That's all.
If you want to move the story forwards, go to Snape's classroom, where you'll have to win another occlumency minigame. You'll also find out that Fred and George are waiting for you on the 5th floor.
The Patil twins: Silencing Umbridge
You might have noticed Umbridge is using new speakers to control students. You have to take care of that. After talking to sisters, place two nearby benches under the speaker, climb onto them and pour the potion over it.
Do the same with all 5 remaining speakers. There's no need to write about all of them.
Fred and George: Finest hour
Surprise: you're able to control the twins for a while. Start from using the portable swamp (same way as in Luna's task). Now something more interesting: you have to cast Incendio on nearby boxes of fireworks while flying on a broom. Nothing disturbs you, so you shouldn't have any problems. There are plenty of boxes, but you don't have any time limit. The last, big box is in great hall.
Once you control Harry again, run to the castle. There's a fight between Malfoy supported by his friends and your mates taking place in stone circle. Help your friends to defeat enemies.