The Calculation[]
First of all, scaling.
Since the tree is mostly bare, I'm going to have to assume it's species. Since you can see a small bit of foilage on it, I'll assume it's a Callery pear tree. Callery pear is the third most common tree in NY, which I think Gotham is based off of anywho so it make sense. If anyone has any better suggestion, feel free to let me know.
The trees can grow up to 5-8 meters tall, so I'll be using 6.5 as a mid end.
Pixels
Tree (Red) = 194 PX
Building Height (Purple) = 343 PX
Building Width (Green) = 402 PX
Building Frontal Width (White) = 810 PX
North West Room Height (Gray) = 160 PX
North West Room Width (Orange) = 202 PX
North West Room Frontal Width (Hot Pink) = 251 PX
North East Room Height (Pink) = 95 PX
North East Room Width (Brown) = 210 PX
North East Room Frontal Width (Dark Green) = 240 PX
Whew, that was tedious.
Scaling
So we assumed that the tree is 6.5 Meters.
343 / 194 = 1.76804123711
402 / 194 = 2.07216494845
810 / 194 = 4.175257731958763
194 / 160 = 1.2125
202 / 194 = 1.0412371134
251 / 194 = 1.293814432989691
194 / 95 = 2.04210526316
210 / 194 = 1.0824742268
240 / 194 = 1.237113402061856
Sizes and Volumes
6.5 *1.76804123711 = 11.492268041215 Meters
6.5 *2.07216494845 = 13.469072164925 Meters
6.5 * 4.175257731958763 = 27.13917525773196 Meters
This will serve as the volume for the main school building.
Volume = Rectangle
Volume = length * Width * Height (No Order Needed)
11.492268041215 * 13.469072164925 * 27.13917525773196 = 4200.87802907 m^3
Now for the small rooms at top. Lets start with the eastern one.
6.5 / 1.2125 = 5.360824742268041 Meters
6.5 * 1.0412371134 = 6.7680412371 Meters
6.5 * 1.293814432989691 = 8.409793814432992 Meters
Using Rectangle once again.
8.409793814432992 * 6.7680412371 * 5.360824742268041 = 305.1265184786321 m^3
Now for the western one.
6.5 / 2.04210526316 = 3.182989690718368 Meters
6.5 * 1.0824742268 = 7.0360824742 Meters
6.5 * 1.237113402061856 = 8.041237113402064 Meters
You know the drill.
3.182989690718368 * 7.0360824742 * 8.041237113402064 = 180.089761063606 m^3
180.089761063606 + 305.1265184786321 + 4200.87802907 = 4686 m^3
4686 * 1000000 = 4686000000 cm^3
Now to solve for hollowness. I'll assume 80% hollow sense I recall that's OBDs standard hallow. Seems reasonable in this case.
20% of 4686000000 is 937200000 cm^3
Now for busting it.
As we see in this scan. The school was fragmented pretty hard. You see lots of small fragments flying, so I'll be using violent fragmentation here.
937200000 * 69 = 64,666,800,000 Joules, or 15.4 Tons of TNT.
City Block level. Not bad.
...Well...
In hindsight, most of the fragments were rather large, so I guess setting up an end using basic Fragmentation would be fine.
937200000 * 8 = 7,497,600,000 Joules. That is roughly 1.7 Tons of TNT.
Building level+