>>1)語学学校について
>スコアなどについて調べたのですが、平均でどの程度のスコアを保持していないといけないものなのでしょうか。また、そのスコアに達するまでの時間等をご教示願います。
You don't have to take the TOEFL or the IELTS to apply to the ESL school. That's for the students who study English for taking academic education and/or surviving their lives out there. To get an admission to an undergraduate or a graduate program, you need to get a score of at least 550(PBT)/85(iBT) on the TOEFL. If your TOEFL score is less than 450(PBT), you'll probably need to spend at least 9-12 months at the ESL program. If your English is as equivalent to the 2nd Grade for the STEP English or 550-600 in TOEIC, you may be able to finish the ESL earlier than those who don't reach to that level. Still you need to hang in there for awhile (3-6 months), though.
>そして、語学学校へ進んだ場合、日本人はどの程度そこで過ごすのでしょうか。カリキュラムによって違うと思いますが、目安でかまわないので教えていただきたいです。
I don't know how long most Japanese students stay in the ESL program prior to their study. It depends on schools and their commitment. Students who came to the US after finishing high school in Japan usually take at least 2 semesters before they study at an undergraduate program. They should be able to finish within 6-9 months, if they have some knowledge about grammar, vocabulary, and sentence. But some Japanese students fall into the cracks--they end up staying more than 1 year or a 1.5 year at the ESL program until they get run out of money.
>2)大学への進学
>語学学校を経て、大学進学の際はどのような試験を受けるのでしょうか。
You need to take the TOEFL and get a minimum score for applying to an undergraduate program.
>進学希望は、未だ具体的には考えていないのですが、アメリカでは大学から同大学院への進学が一般的なのでしょうか。
There are more students who go to graduate school in the US than in Japan. Although the vast majority of students get a job after finishing the degree, some of them come back to school for graduate degree in few years.