Interest expense. Interest expense was $0.8 million for the three months ended June 30, 2021, a decrease of $0.3 million, as compared to interest expense of $1.1 million for the three months ended June 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 an average interest rate reduction of approximately 42 basis points from the second quarter of 2020 to the second quarter of 2021.
Investment interest and other. Investment interest and other for the three months ended June 30, 2021 was $0.2 million, a decrease of $0.2 million, as compared to $0.4 million in the three months ended June 30, 2020. This decrease was primarily attributable to a decline in interest income on excess cash due to lower interest rates.
Income tax expense. Income tax expense for the three months ended June 30, 2021 was $15.0 million, a decrease of $0.3 million, or 2.0%, as compared to income tax expense of $15.3 million for the three months ended June 30, 2020. This decrease was the result of a decrease in our effective tax rate between periods partially offset by higher taxable income. The lower effective tax rate was primarily due to favorable adjustments as a result of the completion of several state audits. Our effective tax rate was 23.3% during the second quarter of 2021 compared to 24.6% during the second quarter of 2020.
Net income. Our net income for the three months ended June 30, 2021 was $49.5 million, an increase of $2.5 million, or 5.2%, as compared to $47.0 million for the three months ended June 30, 2020, due to the factors discussed above.
Six Months Ended June 30, 2021 Compared to Six Months Ended June 30, 2020
Service revenue. Our service revenue for the six months ended June 30, 2021 was $438.4 million, an increase of $31.0 million, or 7.6%, as compared to service revenue of $407.4 million for the six months ended June 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 101,808 at June 30, 2021 as compared to 98,326 at June 30, 2020. University partner enrollments at our off-campus classroom and laboratory sites were 4,210, an increase of 13.2% over enrollments at June 30, 2020, which includes 176 GCU students at June 30, 2021. Enrollments at GCU grew to 97,774 at June 30, 2021, an increase of 3.3% over enrollments at June 30, 2020. GCU enrollment declines between March 31 and June 30 of each year as ground enrollment at GCU at June 30 of each year only includes traditional-aged students taking Summer school classes, which is a small percentage of GCU’s traditional-aged student body and professional studies students. The Spring semester for GCU’s traditional-aged student body ends near the end of April each year. GCU also had a decline in the year over year growth rate of its online students between March 31, 2021 to June 30, 2021 (see – Impact of COVID-19 above). The increase in revenue per student is primarily due to the service revenue impact of the increased room, board, fee and other ancillary revenues at GCU in the six months ended June 30, 2021 as compared to the six months ended June 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 twelve 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 as compared to 23 at June 30, 2020. 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 six months ended June 30, 2021 were $65.7 million, an increase of $12.3 million, or 23.0%, as compared to technology and academic services expenses of $53.4 million for the six months ended June 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 $8.9 million, $2.8 million and $0.6 million, respectively. These increases were primarily due to increased headcount to support our 27 university partners, and their increased enrollment growth, tenure-based salary adjustments, an increase in benefit costs and the increased number of