$4.7 million in personnel and related benefit costs, including stock-based compensation, and $5.3 million in professional fees and marketing expenses. Selling, general, and administrative expenses were 23.2% of revenues during the three months ended December 31, 2023, an increase from 22.3% for the three months ended December 31, 2022.
Other income (expense), net. Other income, net for the three months ended December 31, 2023 was $6.5 million as compared to $4.0 million in the three months ended December 31, 2022. The increase in other income, net was primarily due to the increase in our investments in marketable securities and the returns on those investments year over year.
Income tax expense. Income tax expense was $22.2 million for the three months ended December 31, 2023, or 24.9% of income before income taxes, as compared to an expense of $18.9 million, or 27.1% of income before income taxes for the same period in the prior year. The decrease in the effective tax rate for the three months ended December 31, 2023 was primarily due to the tax impact of non-deductible compensation and excess tax benefit of stock-based compensation.
Comparison of the Six Months Ended December 31, 2023 and 2022
Revenues. Our revenues for the six months ended December 31, 2023 were $985.0 million, representing an increase of $101.4 million, or 11.5%, from $883.6 million for the same period in the prior year. General Education revenues increased $66.8 million, or 12.2%, year over year. The increase in General Education revenues was primarily due to the 7.5% increase in enrollments, school mix (distribution of enrollments by school), and other factors. Career Learning revenues increased $34.6 million, or 10.3%, primarily due to a 10.4% increase in enrollments and school mix.
Instructional costs and services expenses. Instructional costs and services expenses for the six months ended December 31, 2023 were $611.0 million, representing an increase of $27.2 million, or 4.7%, from $583.8 million for the same period in the prior year. This increase in expense was due to the timing of hiring of personnel and salary increases. Instructional costs and services expenses were 62.0% of revenues during the six months ended December 31, 2023, a decrease from 66.1% for the six months ended December 31, 2022.
Selling, general, and administrative expenses. Selling, general, and administrative expenses for the six months ended December 31, 2023 were $286.5 million, representing an increase of $26.1 million, or 10.0% from $260.4 million for the same period in the prior year. The increase was primarily due to an increase of $13.2 million in bad debt expense, $10.8 million in personnel and related benefit costs, including stock-based compensation, and $1.1 million in professional fees and marketing expenses. Selling, general, and administrative expenses were 29.1% of revenues during the six months ended December 31, 2023, a decrease from 29.5% for the six months ended December 31, 2022.
Other income (expense), net. Other income, net for the six months ended December 31, 2023 was $11.7 million as compared to $5.0 million in the six months ended December 31, 2022. The increase in other income, net was primarily due to the increase in our investments in marketable securities and the returns on those investments year over year.
Income tax expense. Income tax expense was $23.7 million for the six months ended December 31, 2023, or 24.9% of income before income taxes, as compared to an expense of $11.4 million, or 28.8% of income before income taxes for the same period in the prior year. The decrease in the effective tax rate for the six months ended December 31, 2023 was primarily due to the tax impacts of non-deductible compensation and Tallo transaction, which were partially offset by the decrease in excess tax benefit of stock-based compensation.
Liquidity and Capital Resources
As of December 31, 2023, we had net working capital, or current assets minus current liabilities, of $848.0 million. Our working capital includes cash and cash equivalents of $354.4 million and accounts receivable of $509.6 million. Our working capital provides a significant source of liquidity for our normal operating needs. Our accounts receivable balance fluctuates throughout the fiscal year based on the timing of customer billings and collections and tends to be highest in our first fiscal quarter as we begin billing for students. In addition, our cash and accounts receivable were significantly in excess of our accounts payable and short-term accrued liabilities at December 31, 2023.
During the first quarter of fiscal year 2021, we issued $420.0 million aggregate principal amount of 1.125% Convertible Senior Notes due 2027 (“Notes”). The Notes are governed by an indenture (the “Indenture”) between us and U.S. Bank National Association, as trustee. The net proceeds from the offering of the Notes were approximately