Instructional costs and services expenses were 62.7% of revenues during the three months ended March 31, 2023, a decrease from 63.3% for the three months ended March 31, 2022.
Selling, general, and administrative expenses. Selling, general, and administrative expenses for the three months ended March 31, 2023 were $103.1 million, representing an increase of $8.9 million, or 9.4% from $94.2 million for the same period in the prior year. The increase was primarily due to an increase of $7.5 million in personnel and related benefit costs. Selling, general, and administrative expenses were 21.9% of revenues during the three months ended March 31, 2023, a decrease from 22.3% for the three months ended March 31, 2022.
Income tax expense. Income tax expense was $19.5 million for the three months ended March 31, 2023, or 26.0% of income before income taxes, as compared to an expense of $16.7 million, or 28.0% 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 March 31, 2023 was primarily due to the impact of non-deductible compensation and the adjustment to the reserve related to the capital losses that resulted from the Tallo transaction.
Comparison of the Nine Months Ended March 31, 2023 and 2022
Revenues. Our revenues for the nine months ended March 31, 2023 were $1,353.9 million, representing an increase of $122.4 million, or 9.9%, from $1,231.5 million for the same period in the prior year. General Education revenues decreased $99.5 million, or 10.6%, year over year. The decrease in General Education revenues was primarily due to the 22.3% decrease in enrollments, school mix (distribution of enrollments by school), and other factors. Career Learning revenues increased $221.9 million, or 75.0%, primarily due to a 57.5% increase in enrollments and school mix.
Instructional costs and services expenses. Instructional costs and services expenses for the nine months ended March 31, 2023 were $878.9 million, representing an increase of $76.2 million, or 9.5%, from $802.7 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 64.9% of revenues during the nine months ended March 31, 2023, a decrease from 65.2% for the nine months ended March 31, 2022.
Selling, general, and administrative expenses. Selling, general, and administrative expenses for the nine months ended March 31, 2023 were $363.4 million, representing an increase of $45.1 million, or 14.2% from $318.3 million for the same period in the prior year. The increase was primarily due to an increase of $23.5 million in professional services and marketing expenses and $21.9 million increase in personnel and related benefit costs, including stock-based compensation, partially offset by a decrease of $3.0 million in operating lease expense. Selling, general, and administrative expenses were 26.8% of revenues during the nine months ended March 31, 2023, an increase from 25.8% for the nine months ended March 31, 2022.
Income tax expense. Income tax expense was $30.9 million for the nine months ended March 31, 2023, or 27.0% of income before income taxes, as compared to an expense of $29.8 million, or 27.3% of income before income taxes for the same period in the prior year. The decrease in the effective tax rate for the nine months ended March 31, 2023 was primarily due to the impact of non-deductible compensation, partially offset by the reserve related to the capital losses that resulted from the Tallo transaction.
Liquidity and Capital Resources
As of March 31, 2023, we had net working capital, or current assets minus current liabilities, of $722.6 million. Our working capital includes cash and cash equivalents of $373.7 million and accounts receivable of $474.7 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 March 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 $408.6 million after deducting the underwriting fees and other expenses paid by the Company. The Notes bear interest at a rate of 1.125% per annum, payable semi-annually in arrears on March 1st and September 1st of each year, beginning on