The following are my suggestions:
Chinese1-A (never learn Chinese before):
"你好!" would be a good start (you don't have to consider the version for the beginning level, they learn basic phrases, the one thing I have to remind you that you cannot teach your own Chinese (Chinese you speak in Taiwan). You might not be used to the phrases in the textbook, because you didn't grow up in China. You have to be aware of you also should teach students what people say in China.)
“遠東少年中文” is also good, especially their CD-ROM exercise. But I don’t like students’ workbook. They should give students more Chinese characters. The workbook only let students rely on “pinyin” more.
“快樂漢語” , you can find some video clips that will make students feel interesting. It is designed to 6years old to 15 years old students.
Chinese 1-B (Have learned Chinese for 2-3 years): You can check “你好!” level 3 or 4.
“中文聽說讀寫” level 1 part 2. “遠東少年中文” level 2 or 3
Chinese 2(Have learned Chinese for 1 yr) “你好!” level 2“遠東少年中文” level 2
Chinese 3( 2yr) “中文聽說讀寫” level 1 part 2, “遠東少年中文” level 2
For high school levels, most of school use “你好!”, “中文聽說讀寫”, “遠東少年中文”. If your school has enough computers for each students, you also can use “IQ Chinese”. All the materials above have many levels.
There is no perfect textbook. You have to think about what is your program’s long term goal and your own short term goal. If I were you, I will use “你好!” series in the beginning for different levels. Remember students always need to review what they had learned, and give them supplementary materials. In this way, you will have better ideas of pipe-line materials. “你好!” sentence patterns are easy but vocabulary is great and useful. Students can do lots of simple exercise to be comfortable with. It will be easy for you to prepare.
If your students are independent school students and really good on academic, you can choose “中文聽說讀寫” series. There are lots of on-line resources you can use, and the workbook is well designed for students practicing their four skills—reading, writing, listening and speaking. Depends on your pedagogy students might feel slow on the progress. You also can use “新實用漢語”, but as I mentioned before I don’t like the text have to go with pinyin. I believe all the American students would be able to write and read in Chinese characters as long as teacher give them good instruction and plenty opportunity to practice. The important thing is not how much you teach them is how well you teach them and give them confidence.