The third-grade students who participated in the 11th "Math Walk" and entered the finals were arranged in a square formation, with 15 remaining, and at least 18 more people are needed to form a larger square formation. There are $\qquad$ third-grade students participating in the 11th "Math Walk" competition.
Square array problem.
Based on the original square array, adding one row and one column requires $15+18=33$ people; thus, the original square array has $(33-1) \div 2=16$ people on each side;
Grade 3 has $16 \times 16+15=271$ people participating in the 11th "Math Walk" final.