$0.1 million, as compared to $14.9 million for the three months ended September 30, 2020. The Secured Note bears interest at 6% annually, and GCU makes monthly interest payments.
Interest expense. Interest expense was $0.7 million for the three months ended September 30, 2021, a decrease of $0.2 million, as compared to interest expense of $0.9 million for the three months ended September 30, 2020. The decrease in interest expense was primarily due to a decline in the average credit facility outstanding balance between periods due to paydowns of the credit facility during the past twelve months and a slight decline in the average interest rate between periods.
Investment interest and other. Investment interest and other for the three months ended September 30, 2021 and 2020 was $0.2 million for both periods.
Income tax expense. Income tax expense for the three months ended September 30, 2021 was $12.2 million, a decrease of $0.9 million, or 7.4%, as compared to income tax expense of $13.1 million for the three months ended September 30, 2020. This decrease was the result of a decrease in our taxable income partially offset by a slight increase in our effective tax rate between periods. Our effective tax rate was 20.3% during the third quarter of 2021 compared to 20.2% during the third quarter of 2020.
Net income. Our net income for the three months ended September 30, 2021 was $47.7 million, a decrease of $4.3 million, or 8.4%, as compared to $52.0 million for the three months ended September 30, 2020, due to the factors discussed above.
Nine Months Ended September 30, 2021 Compared to Nine Months Ended September 30, 2020
Service revenue. Our service revenue for the nine months ended September 30, 2021 was $645.2 million, an increase of $39.4 million, or 6.5%, as compared to service revenue of $605.8 million for the nine months ended September 30, 2020. The increase year over year in service revenue was primarily due to year over year increases in university partner enrollments and in revenue per student. Partner enrollments totaled 118,832 at September 30, 2021 as compared to 117,772 at September 30, 2020. University partner enrollments at our off-campus classroom and laboratory sites were 5,652, an increase of 12.1% over enrollments at September 30, 2020, which includes 286 GCU students at September 30, 2021. Enrollments at GCU grew to 113,466 at September 30, 2021, an increase of 0.6% over enrollments at September 30, 2020. Enrollments for GCU ground traditional students were 23,628 at September 30, 2021 up from 22,363 at September 30, 2020 primarily due to a 9.5% increase in traditional ground students between years. GCU’s ground traditional students residing on campus in GCU’s residence halls increased from 11,441 in the Fall of 2020 to 15,570 in the Fall of 2021, an increase of 36.1%, representing approximately 65.9% of GCU’s ground traditional students. GCU had a decline in its working adult students (online and professional studies) between September 30, 2020 to September 30, 2021 (see – Impact of COVID-19 above). The increase in revenue per student between years is primarily due to the service revenue impact of the increased room, board and other ancillary revenues at GCU in the nine months ended September 30, 2021 as compared to the nine months ended September 30, 2020 (see - Impact of COVID-19 above) and the growth in the enrollment for students at off-campus classroom and laboratory sites. Service revenue per student for off-campus classroom and laboratory sites generates a significantly higher revenue per student than we earn under our agreement with GCU, as these agreements generally provide us with a higher revenue share percentage, the partners have higher tuition rates than GCU and the majority of their students are studying in the Accelerated Bachelor of Science in Nursing program and take more credits on average per semester. The ten new off-campus classroom and laboratory sites opened in the past 15 months, partially offset by the non-renewal of the contract with a university partner with two sites in the first quarter of 2021 increased the total number of these sites to 31. In addition, we generated slightly more revenues in 2020 as compared to the same period in 2021 due to 2020 being a Leap Year and thus providing an extra day of revenue in 2020 as compared to 2021.
Technology and academic services. Our technology and academic services expenses for the nine months ended September 30, 2021 were $101.3 million, an increase of $17.1 million, or 20.3%, as compared to technology and academic services expenses of $84.2 million for the nine months ended September 30, 2020. This increase was primarily due to increases in employee compensation and related expenses including share-based compensation, in occupancy and depreciation including lease expenses, and in technology and academic supply costs of $13.4 million, $3.2 million and