the same period in the prior year. The decrease in the effective tax rate for the three months ended December 31, 2021 was primarily due to the impact of non-deductible compensation and the excess tax benefit from stock-based compensation.
Comparison of the Six Months Ended December 31, 2021 and 2020
Revenues. Our revenues for the six months ended December 31, 2021 were $809.7 million, representing an increase of $62.6 million, or 8.4%, from $747.1 million for the same period in the prior year. General Education revenues decreased $8.3 million, or 1.3%, year over year. The decrease in General Education revenues was primarily due to the 9.8% decrease in enrollments, school mix (distribution of enrollments by school), and other factors. Career Learning revenues increased $70.9 million, or 59.4%, primarily due to a 37.8% increase in enrollments, school mix, as well as from the acquisition of MedCerts and Tech Elevator.
Instructional costs and services expenses. Instructional costs and services expenses for the six months ended December 31, 2021 were $535.8 million, representing an increase of $48.0 million, or 9.8%, from $487.8 million for the same period in the prior year. This increase in expense was due to continued hiring of personnel in growth states and salary increases in some states. Instructional costs and services expenses were 66.2% of revenues during the six months ended December 31, 2021, an increase from 65.3% for the six months ended December 31, 2020.
Selling, general, and administrative expenses. Selling, general, and administrative expenses for the six months ended December 31, 2021 were $224.0 million, representing an increase of $15.2 million, or 7.3% from $208.8 million for the same period in the prior year. The increase was primarily due to an increase of $7.2 million in licensing fees, $6.3 million in professional services and marketing expenses, and $1.1 million in personnel and related benefit costs, including stock-based compensation. Selling, general, and administrative expenses were 27.7% of revenues during the six months ended December 31, 2021, a decrease from 27.9% for the six months ended December 31, 2020.
Income tax expense. Income tax expense was $13.0 million for the six months ended December 31, 2021, or 26.5% of income before income taxes, as compared to an expense of $8.3 million, or 18.2% of income before income taxes for the same period in the prior year. The increase in the effective tax rate for the six months ended December 31, 2021 was primarily due to the impact of non-deductible compensation and the excess tax benefit from stock-based compensation.
Liquidity and Capital Resources
As of December 31, 2021, we had net working capital, or current assets minus current liabilities, of $577.9 million. Our working capital includes cash and cash equivalents of $257.0 million and accounts receivable of $430.4 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, 2021.
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 March 1, 2021. The Notes will mature on September 1, 2027. In connection with the Notes, we entered into privately negotiated capped call transactions (the “Capped Call Transactions”) with certain counterparties. The Capped Call Transactions are expected to cover the aggregate number of shares of the Company’s common stock that initially underlie the Notes, and are expected to reduce potential dilution to the Company’s common stock upon any conversion of Notes and/or offset any cash payments the Company is required to make in excess of the principal amount of converted Notes. The upper strike price of the Capped Call Transactions is $86.174 per share. The cost of the Capped Call Transactions was $60.4 million and was recorded within additional paid-in capital.
Before June 1, 2027, noteholders will have the right to convert their Notes only upon the occurrence of certain events. After June 1, 2027, noteholders may convert their Notes at any time at their election until two days prior to the maturity date. We will settle conversions by paying cash up to the outstanding principal amount, and at our election, will settle the conversion spread by paying or delivering cash or shares of our common stock, or a combination of cash and shares of our