Is it reliable for two high school students (girls) to book a free trip to Japan on Ctrip?

Not reliable. First of all, it is impossible for 18 years old to apply for a Japanese individual single tourist visa without a guardian. Secondly, even if you are above 18, you'd better consult in advance what the free travel you reported contains and what materials you need. Generally speaking, free travel is the cost of airport wine and visa. What information does a visa need to provide? If you can't provide all the information, you risk being refused a visa. Once the visa is rejected, there will be some related problems, such as how to refund the travel expenses you reported and how much. It is recommended to ask for relevant information in detail before booking.