We solve the (ungimmicked) crossword puzzle, and find the clues to help find the theme: the title, 9-A and 98-A. Looking for something mathematical, we find 4 math classes in the grid (à la a word search).
The classes are highlighted in the grid below: TRIGONOMETRY, ALGEBRA, STATISTICS, and GEOMETRY.
1 T | 2 C | 3 M | 4 P | 5 L | 6 E | 7 B | 8 E | 9 M | 10 A | 11 T | 12 H | ||||||
13 B | R | A | I | 14 N | 15 B | E | A | T | I | T | 16 P | I | L | O | T | ||
17 P | A | R | T | A | 18 O | R | G | A | N | S | 19 S | L | A | N | T | ||
20 A | M | I | 21 G | 22 W | B | 23 O | L | D | 24 B | A | D | R | E | P | |||
25 P | B | 26 S | 27 E | R | 28 R | S | 29 I | 30 T | L | L | 31 M | R | S | ||||
32 B | I | 33 G | T | O | E | 34 R | U | M | 35 S | ! | |||||||
36 R | E | L | O | 37 S | M | 38 H | 39 B | Y | E | 40 I | 41 T | 42 T | ! | ||||
43 C | O | A | L | G | 44 A | S | 45 E | 46 A | R | N | 47 P | E | R | 48 U | |||
49 R | A | N | S | O | M | 50 O | X | T | E | A | 51 M | 52 N | A | S | |||
53 O | M | S | 54 T | 55 I | N | E | A | R | 56 S | 57 E | 58 A | N | C | E | |||
59 W | E | E | 60 D | 61 C | M | D | R | 62 Y | U | P | P | I | E | S | |||
63 R | A | E | 64 F | O | E | 65 I | 66 R | V | 67 P | T | S | D | ! | ||||
68 L | 69 Y | N | N | 70 V | E | 71 S | S | E | L | ! | |||||||
72 P | 73 S | 74 U | 75 O | M | S | 76 K | 77 A | S | T | I | 78 D | E | 79 M | ! | |||
80 L | I | N | 81 G | U | A | 82 N | 83 I | N | 84 T | A | 85 T | 86 S | N | 87 L | |||
88 A | D | I | E | U | 89 T | I | B | I | 90 A | E | 91 A | 92 T | S | E | A | ||
93 T | R | O | O | P | 94 I | F | I | T | I | S | 95 B | O | O | M | S | ||
96 T | A | N | S | 97 C | E | S | A | R | 98 O | N | E | ! |
The clue for 98-A tells us the one missing from this list is the answer: CALCULUS.