To find the percentage of first-grade students at the assembly, you can use the formula:
\[ \text{Percentage} = \left( \frac{\text{Number of first-graders}}{\text{Total number of students}} \right) \times 100 \]
Substituting in the values:
\[ \text{Percentage} = \left( \frac{19}{76} \right) \times 100 \]
First, calculate \( \frac{19}{76} \):
\[ \frac{19}{76} = 0.25 \]
Now, multiply by 100:
\[ 0.25 \times 100 = 25 \]
Thus, the percentage of the students at the assembly who were first-graders is:
\[ \boxed{25%} \]