What language do they speak in Singapore schools?

English is the main language spoken in Singapore, and almost all schools use it as medium of instruction (except of course in classes to learn another language).
The only exceptions are some international or niche schools: The Japanese School teaches in Japanese, the French School in French, etc.

beppi wrote:

English is the main language spoken in Singapore,.

Do schools there have Mandarin and/or Malaysian as a first or second language as part of their curriculum, or would those be optional?

In public schools (as opposed to the international schools the OP refered to), all kids have compulsory "mother tongue" lessons - Chinese in Mandarin, Malays in Bahasa and Indians in Tamil - but all other subjects are taught in English. Foreign kids (which are only rarely admitted to public schools) must choose one of the languages.

English:) and an option to take on a second language - Mandarin, Malay or Tamil.

Martenkaevats wrote:

It is more like a general question based on all schools in Singapore and their preferred choice of language within. Hi, My name is Marten Kaevats and living in Singapore last 5 years and I am an education consultant and I am here to provide my answer on this. Well, most of the public schools in Singapore follow the local Chinese or Mandarin language with having some international language options. But, if you refer to American [link under review] in Singapore, then it has English as the primary language for all the students to adopt universally speaking language skills and can settle anywhere across the globe with ease.

This is patently nonsense!
If you are an education consultant, you are a really bad one.
There is NO school in Singapore teaching in Chinese (other than of course classes to learn Chinese language). The difference between the American (meaning USA) school and all other ones is that the former has LESS language teaching and thus prepares pupils less for settling across the globe (other than English speaking countries like Singapore).