Are my sentences correct?

'No-one' refers to 'not one' and therefore you must agree it with the singular verb.
'One is', therefore 'No-one is'.
Also note that it is hyphenated.

No-one from our college is able to join the marathon today.
No-one is able to join the meeting today due to network issues.

Chandrasekhar Reddyjoin the marathon today.

Do you mean run, watch, register for, or be at the marathon?

Chandrasekhar Reddyjoin the meeting

Do you mean attend the meeting?

yes teechr.

Chandrasekhar Reddyyes teechr.

And how about my first question?

David HattonAlso note that it is hyphenated.

There is no need for a hyphen in no one.

In British English it would be odd to write 'no one' rather than 'no-one'. I gather that the US treats it differently!

No one (without a hyphen) is the standard BrE version as far as I know. That's the form in my Oxford Concise English Dictionary, the online Cambridge ALD and the online Macmillan (BrE version).

David HattonIn British English it would be odd to write 'no one' rather than 'no-one'. I gather that the US treats it differently!

I think both the British and Americans write 'no one'. I believe few write 'no-one'.

PS: cross-posted with fivejedjon.

I agree that the OED still has it as 'no one' but in common usage I've rarely seen it written in the UK without the hyphen. It seems to me to be the more common version in this country. A quick internet search shows the BBC, The Independent and The Daily Mail all using it hyphenated. Whether it's accepted in-house style in those organisations I don't know, but I can honestly say I rarely - if e

This https://books.google.com/ngrams/graph?content=No+one%2Cno-one%2Cnobody&year_start=1969&year_end=2009&corpus=18&smoothing=3&share=&