To calculate the percentage of seats filled by the 6th graders, we can use the formula:
\[ \text{Percentage} = \left( \frac{\text{Number of seats filled}}{\text{Total number of seats}} \right) \times 100 \]
Given:
- Number of seats filled by the 6th graders = 1320
- Total number of seats in the theater = 1500
Substituting the values into the formula:
\[ \text{Percentage} = \left( \frac{1320}{1500} \right) \times 100 \]
Calculating the fraction:
\[ \frac{1320}{1500} = 0.88 \]
Now, multiplying by 100 to convert to a percentage:
\[ 0.88 \times 100 = 88 \]
Therefore, the percentage of the seats in the theater that were filled by the 6th graders is
\[ \boxed{88%} \]