Sure! Here is a Python function to generate test cases for the "School of Killifish" problem:

```python
import random

def tcgen():
    # Constraints
    max_r_c = 10**6
    max_q = 10**4
    max_dangerousness = 2**31 - 1
    
    # Generate dimensions r and c such that r * c <= 10^6
    r = random.randint(1, min(1000, max_r_c // 1000))
    c = random.randint(1, min(1000, max_r_c // r))
    
    # Generate the grid with random dangerousness values
    grid = [[random.randint(0, max_dangerousness) for _ in range(c)] for _ in range(r)]
    
    # Generate the number of plans q
    q = random.randint(1, max_q)
    
    # Generate q plans
    plans = []
    for _ in range(q):
        r1 = random.randint(0, r - 1)
        c1 = random.randint(0, c - 1)
        r2 = random.randint(r1, r - 1)
        c2 = random.randint(c1, c - 1)
        plans.append((r1, c1, r2, c2))
    
    return r, c, q, grid, plans

# Function to print the generated test case in the required format
def print_test_case(r, c, q, grid, plans):
    print(r, c, q)
    for row in grid:
        print(' '.join(map(str, row)))
    for plan in plans:
        print(' '.join(map(str, plan)))

# Example usage:
if __name__ == "__main__":
    r, c, q, grid, plans = tcgen()
    print_test_case(r, c, q, grid, plans)
```

This code defines a function `tcgen` that generates random test cases for the "School of Killifish" problem. The function ensures the constraints given in the problem statement are respected. It generates a grid with random dangerousness values and a list of subgrid plans. The `print_test_case` function prints the generated test case in the format specified by the problem statement.

You can run the `if __name__ == "__main__":` block to see an example of a generated test case. This generator can be used to create diverse test cases for thorough testing of the solution to the problem.