For tax year 2017, you can find the list of counties and their school districts at the following location:
https://tax.iowa.gov/expanded-instructions/school-district-surtax-emergency-medical-services-surtax-...
For Polk county the available school districts and their corresponding surtax rates are:
0261 ANKENY ............................... 0%
0472 BALLARD.............................. 2%
0720 BONDURANT-FARRAR........ 3%
0981 CARLISLE............................. 0%
1350 COLLINS-MAXWELL ............ 0%
1576 DALLAS CENTER-GRIMES.. 0%
1737 DES MOINES........................ 0%
3231 JOHNSTON........................... 0%
3942 MADRID ................................ 6%
4779 NORTH POLK....................... 1%
5319 PCM ...................................... 5%
5805 SAYDEL ................................ 0%
6101 SOUTHEAST POLK.............. 5%
6579 URBANDALE ........................ 0%
6957 WEST DES MOINES ............ 0%
7110 WOODWARD-GRANGER..... 7%