Penn is at 3300 now, and they don't take out of district students in middle or high school now. (You can still get in elementary school via a lottery). Adams is the "better" school of the SB schools and is close to 100% capacity. I'd assume most of the Clay kids will go to Riley because it's closer and if they absorb Clay, they'll also be close to 100% capacity. Washington will be the challenge to close as it's out on the westside, but nowhere near capacity.
By the way, interesting discussions about school size in this thread. The 6A football classification schools are (largest schools):
1) Carmel 5,327
2) Ben Davis 4,557
3) Warren Central 3,845
4) North Central 3,709
5) Fishers 3,611
6) HSE 3,430
7) Pike 3,387
8) Penn 3,345
9) Elkhart 3,294
10) Avon 3,248
11) Noblesville 3,220
12) Lake Central 3,211
13) Franklin Central 3,206
14) Brownsburg 2,991
15) Crown Point 2,923
16) Lawrence North 2,833
17) Center Grove 2,754
18) Westfield 2,611
19) Homestead 2,502
20) FW Carroll 2,475
21) Southport 2,379
22) Perry Meridian 2,373
23) Portage 2,269
24) Lawrence Central 2,245
25) Zionsville 2,233
26) Lafayette Jeff 2,153
27) Columbus North 2,119
28) Tech 2,111
29) Jeffersonville 2,099
30) Warsaw 2,087
31) Northrop 2,080
32) Cathedral 1,073
Schools close to moving up to 6A in the next round of classifications (within 100 students):
WL Harrison 2,079
Valpo 2,063
Whiteland 2,012
Merrillville 2,003
Decatur Central 2,001
Chesterton 1,995
Goshen 1,980